The cost to manufacture x pumps is

$C(x)=x^2-21x+100$

If the average cost is \$28 per pump then the total cost for x pumps is $28x$

so

$C(x)=x^2-21x+100=28x$

Just rearrange this and apply the quadratic formula to solve for the roots of this equation.

I'm actually seeing two valid answers though one is more realistic than the other.