If a number x is something more than another number y, it means that you can obtain x by adding something to y.
So, if we want to get 4 more than -9, it means that we must add 4 to -9: the number you're looking for is
On a number line, bigger numbers lie on the right, so if you start from -9 you should take 4 steps to the right, and you'll land on a number which is 4 more than -9.
In a number line system addition mean movement to the right will subtraction is movement to the left which simply implies the bigger numbers lie on the right and the smaller number to the left,
so in the case above starting from -9 you should take 4 steps to the right, and you'll land on a number which is 4 more than -9 which is -5.
Since we want to know how far she goes every hour to multiply that by 4 to get how far she goes in 4 hours, we do 17/3.5 to get per hour miles = around 4.86. Multiply that by 4 to get around 19.43 miles