Get a common denominator, and then solve the equation like so:
$$\left({\frac{{\mathtt{4}}}{{\mathtt{5}}}}\right){\mathtt{\,\small\textbf+\,}}\left({\frac{{\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{x}}}{{\mathtt{7}}}}\right) = {\mathtt{6}}$$
$$\left({\frac{{\mathtt{28}}}{{\mathtt{35}}}}\right){\mathtt{\,\small\textbf+\,}}\left({\frac{{\mathtt{20}}{\mathtt{\,\times\,}}{\mathtt{x}}}{{\mathtt{35}}}}\right) = {\mathtt{6}}$$
$${\frac{\left({\mathtt{28}}{\mathtt{\,\small\textbf+\,}}{\mathtt{20}}{\mathtt{\,\times\,}}{\mathtt{x}}\right)}{{\mathtt{35}}}} = {\mathtt{6}}$$
$${\mathtt{28}}{\mathtt{\,\small\textbf+\,}}{\mathtt{20}}{\mathtt{\,\times\,}}{\mathtt{x}} = {\mathtt{210}}$$
$${\mathtt{20}}{\mathtt{\,\times\,}}{\mathtt{x}} = {\mathtt{182}}$$
$${\mathtt{x}} = {\frac{{\mathtt{182}}}{{\mathtt{20}}}}$$
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