Algebra
posted by Benjamin .
There is a table of values for the function f. They read:
x=3, f(x)=4
x=4 f(x)=5
x=5 f(x)=6
x=6 f(x)=3
x=7 f(x)=5
x=8 f(x)=7
What is the value of 2f(5)f(4)?
I did this wrong obviously, but what I did was figure out the function of five from the table (4), and multiplied that by 2 to get 8. I then got the function of 4, which was 3. I subtracted 3 from 8 to get 5, but that was not the correct answer, it was 7 according to my answer key. Where did I go wrong?

when x = 5, f(x) = 6
when x = 4, f(x) = 5
2*6  5
12  5
7