f(-18) means that you must plug -18 in for x and solve like so:
f(x) = -2|-18| + 8
The two bars (| |) on the either side of -18 means "absolute value." This means that you must the number inside the bars positive so...
-2 * 18 + 8
Now to solve this you use the PEMDAS method:
-2 * 18 + 8
-36 + 8
F(-18) = -28
Hope this helped!
~Just a girl in love with Shawn Mendes