just math

To simplify this, you need a common denominator.

Since 4x + 4y  =  4(x + y), a common denominator is:  4(x + y)(x - y)

Solving:  [ 3/(4(x + y)) ] · [ (x - y)/(x - y) ]  +  [ 1/(x - y) ] · [ 4(x + y) ]/[ 4·(x + y) ]

=  [ 3(x - y) ] / [ 4(x + y)(x - y) ]  +  [ 1·4(x + y) ] / [ 4(x + y)(x - y) ]

=  [ 3(x - y) + 4(x + y) ] / [ 4(x + y)(x - y) ]

=  [ 3x - 3y + 4x + 4y ] / [ 4(x + y)(x - y) ]

=  [ 7x + y ] / [ 4(x + y)(x - y) ]

$$\mathrm{ERROR}\mathrm{ERROR}{\mathtt{\,-\,}}{\mathtt{y}} = \mathrm{ERROR}{\mathtt{\,-\,}}\underset{{\tiny{\text{Error: Unknown Identifier}}}}{{\mathtt{y}}}\mathrm{ERROR}$$