13.

Then, the zeros are x=-5 and x=4.
14.

Then, the zeros are x=-6 and x=10.
15.

Then the zeros are x=5 and x=10.
16.

Then the zeros are x=-8 and x=7.
17.

Then the zeros are x=-8 and x=6.
18.

Then there is only one zero at x=6.
19.

Then the zeros are x=-30 and x=-2.
20.

Then the zeros are x=-4 and x=25.