Find the total revenue function: (TR(q))
Compute marginal revenue as the derivative with respect to quantity: (MR(q)=frac{dTR(q)}{dq})
If revenue is given by a table for discrete quantities, compute marginal revenue for a change from (q_1) to (q_2): (MR=frac{TR(q_2)-TR(q_1)}{q_2-q_1})
For consecutive quantities (q) and (q+1): (MR(q)=TR(q+1)-TR(q))
If price is given as a function of quantity (P(q)), use (TR(q)=P(q)cdot q) and then (MR(q)=frac{d}{dq}[P(q)cdot q])
If only a demand curve is given as (P(q)), apply the same step: set (TR(q)=qcdot P(q)) then differentiate to get (MR(q))