[Video] Q4: If f(x) = -2x + 5, what is f(-3x) equal to?

Explanation for Question 4 From the Math (No Calc) Section on the Official Sat Practice Test 4

Now here for number four, we're looking at what is F of negative three 2 X. And in order to do this problem, 3 we have to understand the notation F of X. 4 And we know that on a graph, all points will have an X comma 5 Y value, right? And so the X value you, 6 we see right here, um, in the parentheses and our Y value is essentially 7 this whole thing that F of X is basically just a fancy way to 8 say Y. And so what that means is that to solve for your Y 9 value, you just plug in whatever's inside of the parentheses. 10 For example, if we were talking about X, F of two, 11 that would essentially mean that X is equal to two, 12 and we would plug in to wherever we saw an X, 13 but in this case, we're not asked for, you know, normal number we're asked 14 for negative three X. And so that essentially means that our X has now 15 become negative three X. So when I solve this equation, 16 then for F of negative three X, 17 I need to do negative two. Um, 18 and then oops, negative two. 19 And then instead of writing X, I'll replace it with that negative three X, 20 because that's, what's in my parentheses right here. 21 And then plus five, right? And negative two times negative three, 22 that gives me a positive six. So I have six X plus five, 23 which we see here in answer choice B.