You'll have to set this up as two separate problems. First, we want to take 15% off the original $20 bill. To do this, you're going to multiply 20 by .15, which gives you $3. So, 15% off $20 is $17.
Now, we want to find 7% of the new bill, so we're going to multiply 17 by .07 to get $1.19, and add it to the $17, making the final bill $18.19. You could also multiply $17 by 1.07 to get your final total without the extra step in the middle, but only do that once you're comfortable with the math.