You want to find 2 values, a and b, and you are given two points, (10, 80) and (30, 20). Setting x= 10, y= 80 in \(\displaystyle y= ab^x\) gives the equation \(\displaystyle 80= a(b^{10})\). Do the same with x= 30, y= 20 to get a second equation so that you have two equations to solve for the two unknowns.
(What happens if you divide one equation by the other?)