System : substitution

Y=x+9

3x+8y=-5

3x+8 (x+9) ＝-5

3x+8x+72＝-5

11x+72=-5 minus 72 at both side

11x＝-77 divide 11 at both side

x＝-7

3x + 8 (x + 9) = - 5

3x + 8x + 72 = - 5

11x + 72 = - 5

11x + 72 - 72 = - 5 - 72

11x = - 77

11x/11 = - 77/11

x = - 7

All you have to do for this equation is to substitute the y in the second equation with what y equals to. Then you distribute and combine like-terms.