Astudent earns a 90 on a test for missing one question incorrectly and earns a 70 for answering three incorrectly. what is the slope?

I got -10

1. create two ordered pairs based on the data so (1, 90) and (3, 70)

2. plug into the slope formula so (70 - 90)/(3 - 1)

then it's -20/2, which is -10