Multiply then change into standard form f(x)=(x+8)(x-2)

Use the FOIL method:
First, Outer, Inner, Last

So its basically,
x • x + x • (-2) + 8 • x + 8 • (-2)
which gives you:

x^2 - 2x + 8x - 16

Simplify after:
x^2 + 6x - 16

I forgot which one standard form is so I'm going to give you two forms. (I'm not sure if it's totally correct)

y = x^2 + 6x - 16
y - x^2 = 6x - 16

X=3 and y=4 is the answer