(I just figured out I can use that "LaTeX" thing)
Using ↑↑↑↑ that function up there
So, you want me to find f(2x)-2f(x), huh?
In the above equation, you can just plug in 2x, which yields:
2(2x)^2-5(2x) = 8x^2 - 10x
Now we find 2f(x), which is just 2(2x^2-5x) = 4x^2 - 10x
subtracting 4x^2 - 10x from 8x^2 - 10x, we get 4x^2.