MAXIMUM PROFIT (multiple variables)

Sadly i do have one more i thought the last question would make more sense of this one unfortunately it did not...

So if you could be so kind here it is...

It costs you c dollars each to manufacture and distribute backpacks. If the backpacks sell at x dollars each, the number sold given by

where a and b are certain positive constants.

what selling price will bring a maximum profit?

in my notes to find the max profit one would have to have two seperate functions find their derivatives and have them = to each other... like in my last question.

The issue here is that there is only one function.

also what is supposed to be the marginal revenue

i am sorry if this is obvious and i am just not missing it.. but something does not seem to add up to me.

thank you so very much...again