Stright Line demand to MR=MC

I have been learning PED and straight line demand, I have the first part of the process I think, however, when working out the Marginal cost and total contribution I am having a few difficulties

I get that MR=MC so if you are given a variable cost how to work that through and work out Q, however, the notes I have say substitute Q into the price function then just gives you an answer and I cant see how this was reached please can anyone provide an extra step that I may be missing?

So the equation is: P = 600 – 0.4Q.
Forecast the demand at a given selling price At a price of $300
300 = 600 – 0.4Q
0.4Q = 300 Q = 300/0.4 Quantity demanded (Q) = 750 units per month

MR = 600 – 0.8Q MC = VC so equating MR = MC: 100 = 600 – 0.8Q
So Q = 625
And substituting Q into Price function, P = $350 ??????????????? – how?