Can yall give me the answers for the unsolved ones

No.  But what I'm going to do is write in the values of 'a', 'b', and 'c' in each expression.  Then all you need to do is the multiplication, and you'll have the answer to each one.

13).  -2a = -2(-3)
14).  9b = 9(-4)
15).  ab = (-3) (-4)

16).  -3ac =  (-3) (-3) (5)
17).  -2c²  =  -2 (5²)
18).  abc  =  (-3) (-4) (5)

Others:

1). to 7)., and 9). and 10).  are all correct

8).  3(-2x) = -6x
11).  -6(-3y) = 18y

12).  should be negative

The surface area would be 576 because you multiply all the numbers then multiply that number by 6