Explanation:
($12, 600) <--- 600 shirts sold at $12
($9.75, 810) <--- the supply was short by 210, which makes it 810
For every $1 increase in price the demand decreases by 50, so the slope is -50
For every $1 increase in price the supply increases by 40, so the slope is +40
Demand line is y = -50x + b
600 = -50*12 + b
600 = -600 + b
b = 1200
y = -50x + 1200
The supplhy line is y = 40x + b
810 = 50*9.75 + b
810 = 487.50 + b
= 322.50
y = 40x + 322.50
Q = -50p + 1200 is the demand
Q = 40p + 322.50 is the supply
the equilibrium price is -50p + 1200 = 40p + 322.50
1200 - 322.50 = 90p
p = 9.75
9.75 is the equilibrium price (if it's too long or you don't understand, just delete)