Differentiate R: 8-2x=0 at maximum, so x=4 for maximum revenue. R(4)=2800+32-16=$2816.
Without using calculus, rewrite R:
R(x)=2800+16-(x²-8x+16)=2816-(x-4)², which clearly is maximum at x=4 (because nothing is taken away from the maximum revenue of $2816).