It could be using F as a variable. (FxF)-(Fx8)
It could also be using F as a number in a base 16 or higher number system. i.e. if you were going on base 16(or higher), you use letters to represent numbers past 10, so a=11, b=12, c=13, d=14, e=15, f=16, etc
so it would translate out to (16x16)-(16x8) which equals 128
This is a math problem in hexadecimal, which is a base-16 numeral system. We normally use a base-10 system, meaning we count from zero to nine, then start over again and add a digit. However, we don't have any shapes designated for numbers greater than nine, so we use letters; namely, we use the letters 'a' through 'f.' The letter 'a' represents 10, 'b' represents 11, and so forth. So you would count 0,1,2...8,9,a,b,c,d,e,f,11,12.... and so on.
FF represents 255, and F8 is equal to 7 less than 255 (in decimal terms). 255-248=7.
(15*16 - 15*16) + (15 - 8) = 7