I know the answer but am not sure how to get it a step by st

I know the answer but am not sure how to get it. a step by step answer would really help, thank you

Solution

given x²-4x-21>0

x²-7x+3x-21>0

factors of 21 are 7,3

x(x-7)+3(x-7)>0

(x-7)(x+3)>0

(x+3)(x-7)>0

c)intervals marked by 7 and -3

solution is x<-3, x>7