How do you solve economics equations to find out how to draw graphs?

Suppose demand is give by P = 300 – 0.5Q and supply is given by P = 0.5Q in the canola market, where Q is tones of canola per year. The government sets a price support at P = $300 / tone and purchase any excess supply at that price. In response, as a long run adjustment, farmers switch their crops from wheat to canola, expanding supply to P = 0.25Q.