Answer :
7 - 3 = 4....so there is a $ 4 profit for every unit sold
4x = 3600
x = 3600/4
x = 900....so 900 units have to be sold to earn a profit of $ 3600
4x = 3600
x = 3600/4
x = 900....so 900 units have to be sold to earn a profit of $ 3600