Question 1041311

Given the following revenue and cost functions, find the x-value that makes revenue a maximum.
{{{R(x) = 68x - 2x^2;}}}{{{ C(x) = 21x + 97}}}

The answer is supposedly 17, but no matter what I do I can't figure out how to get it?
<pre>The revenue function is: {{{R(x) = 68x - 2x^2}}}, and the maximum revenue occurs at {{{highlight(matrix(1,7, x = - b/(2a), "======>", x = - 68/(2 * - 2), "======>", x = - 68/- 4, "======>", highlight_green(x = 17)))}}}
That's all!