The quadratic expression (x+4)(x-8) is written in factored form
write the expression in vertex form a(x-h)^2+k
First convert (x+4)(x-8) to quadratic form which makes it x^2-4x-32. From there "Complete the square" which makes it y+32=x^2-4x...y+36=(x-2)^2.
Then you add 36 to both sides to get y=(x-2)^2-36 which is your answer.
Hope it helps!