Question below need help ASAP

Which expression do you get when you eliminate the negative exponents of 4a^2 b^216a^-3 b

Suppose you are choosing a 6-digit personal access code. this code is made up of 4 digits chosen from 1 to 9, followed by 2 letters chosen from a to z. any of these digits or letters can be repeated. find the total number of personal access codes that can be formed. 492,804 341,172 39,917,124 4,435,236

Geometry - finding area of regular polygons and sectors (check my math, i can’t find the error)