Question

How do you solve `72x - 8y = 40` and `y = 9x + 5` using substitution?

Answer 1

This pair of equations is inconsistent; there are no solutions.

Explanation:

To see that there are no solutions:

Given
[1]`color(white)("XXX")72x-8y=40`
[2]`color(white)("XXX")y=9x+5`
substituting `(9x+5)` (from [2]) for `y` in [1]
[3]`color(white)("XXX")72x-8(9x+5)=40`
[4]`color(white)("XXX")-40=40` (which is clearly impossible)