I know part of the answer is (2(x-y))/y(x+y) but the second part asks for "if" im assuming domain? what is if? and what is the answer to if in this equation

There are 15 plates and each plate is either 7 or 12 inches diameter. which systems of equations do you use to find the number of 7 inch plates, x and 12 inch plates, y

Describe the domain and range of y=2(3)^x-2+5

Marla bought a book for $12.95, a binder for $3.49, and a backpack for $44.99. the sales tax rate is 6%. find the amount of tax and the total she paid for these items