Show that x = - 3 is a solution of x2 + 6x + 9 = 0.
If x = - 3 is a solution then it must satisfy the equation
x2 + 6x + 9 = 0
LHS = x2 + 6x + 9
= ( - 3)2 + 6. - 3 + 9
= 9 - 18 + 9
= 18 - 18
= 0 = RHS
Thus x = - 3 is a solution of the equation