The difference of the square of a number and 21 is equal to four times the number find the negative solution

Question

I don’t under Stand any of this

Answer 1

number = n

 

n²  - 21 =  4n

 

n²  -4n -21 = 0

 

(n-7)(n+3) = 0

 

n= 7 and n= -3