Maximum profit,

Is P the price or the profit? Is Q the quantity? Is TC the total cost?

If P is the price, then
revenue r(q) = pq = q(360-q)/20
profit = r(q) - c(q) = q(360-q)/20 - (6q + 1/20 q^2) = -1/10 q^2 + 12q
as usual, the vertex is at q = -b/2a = 12/20
Now find profit for that value of q.