In this case, you have two segments of your piecewise function and you can separate them into two integrals:
∫0-3 3•dx + ∫20 (6 - x2)dx
= [3x]0-3 +[6x - (1/3)x3]20
= 3(0) - 3(-3) + 6(2) - (1/3)(2)3 - [6(0)-(1/3)(0)3]
= 9 +12 + (8/3)
=(27+36-8)/3
= 55/3
≈18.33
James D.
Incorrect problem setup, please see link to get a better picture..my apologies: https://ibb.co/c14QfrW11/17/21