A government agency has estimated that a flood control project has costs and benefits that are parabolic,

Question:

A government agency has estimated that a flood control project has costs and benefits that are parabolic, according to the equation
(Present worth of benefits)2
- 22(Present worth of cost) + 44 = 0
where both benefits and costs are stated in millions of dollars. What is the present worth of cost for the optimal size project?