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.