We have

And we want

Another way to read it is

It should be clear that the solution is 
Is it a picture or an answer or something
APR=18%
Monthly interest = 18/12=1.5%
If Caitlyn paid before due date,
amount liable for interest charges = 375-250 = $125
Interest charge for the month
=$125*1.5%
=$1.88
If a single expression is required, then
interest = (375-250)*0.018/12 = $1.88 to the nearest cent.
Easy there on the caps, partner.
The nine, using the distributive property, distributes itself across the brackets, and so 9 times t is 9t and 9 times -1 is -9.
Therefore, 9(t-1) = 9t -9