Which of the following functions is quadratic? f(x)=3(x-4)(x+3), f(x)=5^2, f(x)=2/x, f(x)=2x^3+3x^2-5

In B 5^2 = 25. There is no x anywhere.
In C f(x) = 2/x is a function of a reciprocal, not a quadratic.
In D f(x) = 2x^3 + 3x^2 - 5 is a cubic. A cubic is not a quadratic. The highest power of a quadratic is x^2

That leaves A. When you expand the brackets, you get
3(x^2 + 3x- 4x - 12)
2(x^2 - x - 12) when you remove the brackets you get
2x^2 - 2x - 24. The highest power of x is x^2. This is your quadratic.

A<<<< answer.

I’m confused what’s happening

Step 1: 6x-10=1; step 2: 6x=11