Answer:
3^(n+1) - 3^n / 3^(n-1) = 6
To get the answer you have to divide the sale price $172.80 by 73%.
So first you turn 73% into a decimal by moving the decimal two places to the left turning it into .73
$172.80 divided by .73 = $236.71
You can check by then multiplying $236.71 with .73
-5x +3 =18
Subtract 3 from each side
-5x= 15
Divide by -5
x= -3