Since P(x)=R(x)-C(x), according to the given information we can write P(x)=(150-2x)x-(120+60x-x² )=-x²+90x-120. Now, since there is an extra of 4 TL tax the modified profit function is P(x)=-x²+(90-4)x-120=-x²+86x-120. The price that maximizes the profit is where the derivative is zero, i.e., P' (x)=-2x+86=0 which is x=43 TL. We insert this value in the price so that p(43)=150-2·43=64 TL.