Question

Suppose that it cost $17 to place a classified advertisement in the newspaper, plus $3 for each line. Then the cost to place an ad x lines long is given by y dollars, where y=3x+17

Answer 1

Whatever it is you need to solve, you plug that number in as x...

For example, an AD 2 lines long would make the equation this:
y=3(2)+17
=6+17
y= $23 for a 2 line ad.