Centimeters, probably, which (for small things) isn't accurate.
Answer:
$153.75
Step-by-step explanation:
$71.75 / 7=$10.25
$10.25 x 15 =$153.75
-15x - 3y = 3
-15x - 3(5x-1) = 3 [input y]
-15x - 15x - 3 = 3 [distribute 3 over 5x and -1]
-30x - 3 = 3 [combine like terms]
-30x = 6 [add 3 to eliminate the negative]
x =

[divide by -30 to get x alone]
x =

[reduce the fraction]
Hopefully this is helpful enough to get you to your answer
Y=-2x-7
Y=mx+b where m is the slope and b is the y-intercept
He would have to pay $560 because if you take 20% off of the computer using the equation 1000 - (1000 * 0.20) you get 800. This means that the 30% coupon is applied to $800, so using the same template as the equation above, we can do 800 - (800 * 0.30) to get a final answer of 560.