5m2n2 + -1(6mn + -4m2n3 + 5) + 8

Snapser is a noob

5m2n2 + -1(5 + 6mn + -4m2n3) + 8
5m2n2 + (5 * -1 + 6mn * -1 + -4m2n3 * -1) + 8
5m2n2 + (-5 + -6mn + 4m2n3) + 8

Still a noob
-5 + 8 + -6mn + 5m2n2 + 4m2n3

Combine like terms: -5 + 8 = 3
3 + -6mn + 5m2n2 + 4m2n3