Substitution method

You have to determine in one equation what x ( or why ) and then sub it into the other equation.

X + why = 4 turns into x = 4 - why , then you sub that value in for x in the other equation.

Y + -(4-y) = 3 which is simplified into why -4 + why = 3 and then simplified more yet again into
2y = 7 or why = 3 1/2

Using the value of why = 3 1/2 you can find out that x = 1/2

Very exciting imo

Personally I think its faster to use elimination method in this question. Although it depends on preference. You can just add the two equations together.

Ex.
x+y=4
+ y+-x=3
=
2y = 7
why = 7/2