9 - 5q = r
Since there are two unknown variable namely q and r.
We can't solve any of the unknown by providing a number. Instead we can
provide the formula on how to look for its corresponding value.
We know that:
r = 9 - 5q
9 - 5q = r
-5q = r - 9
-5q / -5 = (r - 9) / - 5
q = (r - 9) / -5
