y = 3x + 7.....so sub i 3x + 7 in for y in the other equation
x + y = 5
x + 3x + 7 = 5
4x = 5 - 7
4x = -2
x = -2/4
x = -1/2 or -0.5
y = 3x + 7
y = 3(-1/2) + 7
y = -3/2 + 7
y = -3/2 + 14/2
y = 11/2 or 5.5
solution is (-0.5,5.5) or (-1/2,11/2)....depending on if u want it in decimal form or fraction form