To find that x = 6, use the pythagorean theorm, $${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{b}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$
$${\mathtt{a}} = {\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}$$, $${\mathtt{b}} = {\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}$$
$${\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$ replace $${{\mathtt{c}}}^{{\mathtt{2}}}$$ with $${{\mathtt{x}}}^{{\mathtt{2}}}$$
$${\sqrt{{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}}} = {\mathtt{3.027\: \!735\: \!832\: \!268\: \!483\: \!3}}$$
The answer x = 6 is not correct.
To find the area of the triangle us the formula for area of a triangle, $${\mathtt{a}} = {\frac{{\mathtt{1}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}{\mathtt{l}}{\mathtt{\,\times\,}}{\mathtt{w}}$$
$${\mathtt{l}} = {\mathtt{length}}$$, $${\mathtt{w}} = {\mathtt{width}}$$
$${\mathtt{a}} = {\frac{{\mathtt{1}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right){\mathtt{\,\times\,}}\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)$$
$${\frac{{\mathtt{1}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right){\mathtt{\,\times\,}}\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right) = {\mathtt{2.291\: \!796\: \!067\: \!500\: \!630\: \!9}}$$
The area of the triangle is approximately 2.30
To find the perimeter of the triangle, add up all the sides
$${\mathtt{p}} = \left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right){\mathtt{\,\small\textbf+\,}}\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right){\mathtt{\,\small\textbf+\,}}{\sqrt{{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}}}$$
$$\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right){\mathtt{\,\small\textbf+\,}}\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right){\mathtt{\,\small\textbf+\,}}{\sqrt{{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\left({\sqrt{{\mathtt{15}}}}{\mathtt{\,-\,}}{\sqrt{{\mathtt{3}}}}\right)}^{{\mathtt{2}}}}} = {\mathtt{7.309\: \!600\: \!909\: \!545\: \!562\: \!5}}$$
The perimeter of the triangle is approximately 7.31