Need help on the steps to solving problem

I have this problem:

4/21=(3p-4)/7sqrt(p^2+20)

I could use a detail step through process for solving this. I know that p=4 or -44/65

I know that it comes downs to using the quadratic formula or factoring to solve the quadratic equation.

my steps so far have been

if 4/21=(3p-4)/7sqrt(p^2+20) then 4/21*7sqrt(p^2+20)=3p-4

next I get 28sqrt(p^2+20)/21=3p-4

then 4sqrt(p^2+20)/3=3p-4

then I multiple both sides be the LCD (3) to eliminate the fraction

I get 4sqrt(p^2+20)=9p-12)

then I need help from here so I fully understand

I know I need to square both sides and factor but I need to know all the steps

thank you!