If f(x)=x^2-4x, find f(x+trianglex)

Question

The triangle in the test paper is a symbol I just don't know how to put symbol here.
The answer is (x+trianglex)(x+trianglex-4), the problem is I don't know how it became like that.

Answer 1

when you write f(x) = x^2-4x, the "x" is just the name of the argument. You can replace it with whatever you want. So,
f(2) = 2^2 - 4*2
So,
f(x) = x(x-4)
f(x+∆x) = (x+∆x)(x+∆x - 4)

Surely by this time you have learned that ∆ is the Greek letter "delta" and is used to indicate a small change in the variable x.

Answer 2

Oobleck can u come help me

Answer 3

yeah - I'll be right over...