Step-by-step explanation:
We are given the unit cost to produce x number of airplanes as follows;

However, to minimize the unit cost, we need to first take the derivative of the cost function and then find its value at zero.
Thuis is shown below;


Note that for a derivative, the constant term is always equal to zero. We can now simplify what we have above;


We now set this equal to zero and simplify;

Add 150 to both sides;

ANSWER:
Therefore, to minimize the unit cost, 150 engines must be made.