I tried doing these question s and i'm not getting the answers...i;m quite lost can anyone please help?

1) the demand functions is given as P(x)=[(120-(x/20))^0.5], where x is the # of units sold. if the revenue function is given as R(x)= xP(x), then how many units must be sold to max revenue?

2)A company finds a production cost in dollars for x with the function of

c(x)= 14000+ 0.2x + 0.000003x^2. at what level is the average cost a minimum