Explanation:
x + y = 3 ------------ eqn 1
y = -6x + 3 ---------- eqn 2
Substitute -6x + 3 for y in equation 1
x - 6x + 3 = 3
Evaluating we have,
-5x = 3 - 3
Evaluating further,
x = 0
Substitute 0 for x in equation 1
0 + y =3
Make y subject of formula
y = 3 - 0
y = 3
Hope this helped!