I found my answer by using desmos. It is the best app for graphing. I wrote the equation and then I looked for the point 18 on the y axis and then I looked where it was on the x axis
Because since 90 miles is basically counted as your first day, you do the number of days, n, minus 1 and then multiply it by 5 to find the number of miles walked after the first day. Then you add it to 90.
