How do you write a exponential function y=ab^x whose graph passes through the given points (2,18) and (3,54)?

Given the form of the graph, we notice that this is a parabola, plugging in the two values of x and y, 

$18=ab^2$

$54=ab^3$

sikving the system of two equations, we earn:

$a=2$

$b=3$
Therefore the graph is

$y=2*3^x$