To get a common denominator for the two expressions, you need to have the same variables on the bottom. so, you need w^2 and t to be in the bottom.
Multiply each by 1, in the first case, t/t, and you get:
(t/t) (4t/9w^2) = 4t^2/9tw^2
(w/w) (5/6wt) = 5w/6tw^2
Oh, oops, I forgot the number, now multiply each to get 18 in the denominator (18 is the lowest common):
(2/2) (4t^2/9tw^2) = 8t^2/18tw^2
(3/3) (5w/6tw^2) = 15w/18tw^2
Now they are over common denominators.