Please check the original problem in your book or on your worksheet and make sure you've got it written the same here.
How are you getting 3.5?
If you meant 4n - 2(3-n) = -13 then:
4n-2(3-n) = -13
4n - 6 + 2n = -13
6n - 6 = -13
6n = -7
n = -7/6
. . .
If you meant (4n-2)(3-n) = -13 then:
(4n-2)(3-n) = -13
12n - 4n^2 -6 + 2n = -13
-4n^2 + 14n - 6 = -13
-4n^2 + 14n + 7 = 0
( 1)( 7)
Hm. That doesn't look like it will factor nicely. There's this thing you probably haven't been given yet called the quadratic formula. It's for solving this kind of equation. . .
(-14+-sqrt(196+112))/-8
(-14+-sqrt(308))/-8
(-14+-4sqrt(77))/-8
The square root of 77 doesn't work out to a nice, whole number, so it looks like this version of the problem doesn't have