Determine the tax t per unit that must be imposed to obtain the maximum tax revenue. (Hint: Use the solution of output as a function of t from 2(b) to formulate the tax revenue function.)

So output was originally $\displaystyle p = 25-2x$ and cost was $\displaystyle c(x) = 5x+t$

I worked out what the profit maximising value of x was and substituted it in .

$\displaystyle p = 25 - 2(\frac{30+2t}{-4})$

thus $\displaystyle p = 40+t$

But im not sure what I do to formulate the tax revenue function.