I thought maybe you'd like some explanation instead of just the answer,
which is probably in the back of your book.

(a - b)(2a³ - ab + 3b²)

Distribute each term in the first parentheses into the whole parenthesis
on the right:

a(2a³ - ab + 3b²) - b(2a³ - ab + 3b²)

Do the two resulting distributions:

2a4 - a²b + 3ab² - 2a³b + ab² - 3b³

Collect like terms in ab²

2a4 - a²b + 4ab² - 2a³b - 3b³

Edwin