3x-4y=7, for x

3x-4y=7      Add plus 4y to both sides

3x-4y+4y=4y+7        So +4y-4y=0

3x=4y+7      Divide both side by 3

$${\frac{{\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{x}}}{{\mathtt{3}}}}$$  =  $${\frac{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{y}}{\mathtt{\,\small\textbf+\,}}{\mathtt{7}}\right)}{{\mathtt{3}}}}$$     So  $${\frac{{\mathtt{3}}}{{\mathtt{3}}}}$$  cancel each other

$${\mathtt{x}} = {\frac{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{y}}{\mathtt{\,\small\textbf+\,}}{\mathtt{7}}\right)}{{\mathtt{3}}}}$$      This is your answer, though you don't need the brackets and the multiplication sign between the 4 and the y, this is just the way the built in calculator formats it.