We have the equation:
[tex] -7p(p+8)=21 [/tex]
So then we need to distribute -7p to the parentheses:
[tex] -7p^{2}+(-56p)=21 [/tex]
Then we need to set the equation equal to 0, so we must subtract 21 on both sides:
[tex] -7p^{2}-56p-21=0 [/tex]
Then we need to factor this. So we end up with:
[tex] 7(-x^{2}-8p-3)=0 [/tex]
This is as much as this can be factored, so we cannot go any further.