The benefit from performing a certain activity follows $TB(A)=42*A-8\mathrm{.}4*A^2,$ where $A$ is a measure of the level of activity performed.

At the same time, this activity involves a cost which follows

$TC(A)=2*A+1\mathrm{.}6*A^2+15$

Answer, for this activity:

The net benefit, as a function of $A,$ is $NB(A)=$

The marginal benefit, as a function of $A,$ is $MB(A)=$

The marginal cost, as a function of $A,$ is $MC(A)=$

The optimal level of activity is $A^{**}=$

The net benefit at the optimum is $NB(A^{**})=$