Answer:
Step-by-step explanation:
[tex]\sqrt{5x+21}=x+ 3 \\\\Take \ square \\\\\\5x + 21 = (x + 3)^{2}[/tex]
5x + 21 = x² + 2*x*3 + 3²
5x + 21 = x² +6x + 9
x² + 6x+9 -5x - 21 = 0
x² + 6x - 5x + 9 -21 = 0
x² + x - 12 = 0
Sum = 1
Product = -12
Factors = 4 , -3 {4*(-3) = -12 and 4 + (-3) = 1}
x² + 4x - 3x - 12 = 0
x(x + 4) - 3(x +4) = 0
(x + 4)(x - 3) =0
x = - 4 , 3
