Question

System :substitution Y=x+9 3x+8y=-5

Answer 1

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. 

Answer 2

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