Divide everything by 4 getting:
7y=3x+21
then get all of the variables on one side by subtracting 3x:
7y-3x=21
15*0.07= 1.05 so 7% of 15 is 1.05. we add this to the total so 7% tax is going to be a price of $16.05
16x^2=9
x^2=9/18
x=sqrt(9/16)
x=3/4,-3/4
so the root of the quadratic equation are 3/4 and -3/4
