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Follow-up question regarding solving for price and quantity

For monopoly set the price when MC=MR
For calculating MR first write the equation for total revenue (TR)
TR=P*Q = (800-5Q)*Q --------(1)
Differentiate (1) w.r.t. Q to get MR
MR=dTR/dQ = 800 - 10Q----------(2)
MC = 15Q
Equate MC and MR, we get Q = 32
Put this in equation for P to calculate P, P=640

The above response was very helpful, however I am still having trouble figuring out the precise method used for solving for Q. What is the formula used to equate MC and MR?