17.
(3a2 – 2ab + 3b2) - (-a2 – 5ab + 3b2)
18.
5c – [8c – (6 – 3c)]
19.
10x + [3x – (5x – 4)]
20.
3x 2 – [7x- (4x – x2) + 3]
21.
x2 – [ - 3x+ ( 4 – 7x)]