I see you've posted a bunch of problems. I personally don't like to answer problems in bulk like this when I don't see you've tried to answer them. For the other posts, maybe you could show some work to help us out.
So the profit (assuming we make more money than we lose) is revenue - cost.
So we have
Using the given functions this comes out to [tex](6x^2-x^3)-(50+3x^2)]/math]. Now differentiate and set it equal to zero to find a max.