I assumed that a^3*b =a^3b
ab^2 + 3a^3b - ab^2 + ab^2 - a^3b - 4a^3b + 2a^3b - ab^2
Group like terms together:
ab^2 - ab^2 + ab^2 - ab^2 + 3a^3b - a^3b - 4a^3b + 2a^3b
3a^3b - a^3b - 4a^3b + 2a^3b
[3a^3b + 2a^3b] - [a^3b - 4a^3b]
[5a^3b] - [5a^3b] = 0
