Why is the answer a? plzzz explain.

Step-by-step explanation:

f(x) = x^2 + 4x + 4

f(x + a) = (x + a)^2 + 4(x + a) + 4

f(x + a) = x^2 + 2ax + a^2 + 4x + 4a + 4   Now combine like terms and use the distributive property.

f(x + a) = x^2 + (2a + 4)x + a^2 + 4a + 4

We are told that f(x + a) = x^2 - 6x + 9

So that means that

2a + 4 = - 6      Subtract 4 from both sides.

2a = - 6 - 4

2a = - 10          Divide by 2

a = - 10/2

a = - 5.            

But you are not finished. You have to check the last part.

a^2 + 4x + 4 = 9

(a + 2)^2 = 9

Take the square root of both sides

sqrt(a+ 2)^2 = +/- 3

a+2 = 3

a = 1.

Does this work or is it extraneous? It is extraneous because it will not work for  2(a) + 4 = - 6  

You get 6 = - 6 which is not correct. But you did give the second best answer.

a + 2 = - 3

a = - 3 - 2

a = - 5 which agrees with your previous answer.

This is an extremely hard or tricky or sneaky problem. It takes a lot of conniving to get the right answer and 1 is not a left field answer. It just does not happen to be correct.