X+y=5

y=3x+7

x=?

y=?

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