A negative slope means that two variables are negatively related; that is, when x increases, y decreases, and when x decreases, y increases. Graphically, a negative slope means that as the line on the line graph moves from left to right, the line falls.
If Mr. Miller is going to add $10 every week in the account for 7 weeks, all you have to do is do 10 x 7, which is 70, and add that to the $25 that he already has