To solve this proportion, we can first convert both fractions to improper fractions:

2 2/5 = 12/5

3 1/3 = 10/3

Now we can set up the proportion:

(12/5) : (10/3) = x : 3.5

To solve for x, we can cross multiply:

(12/5) 3.5 = (10/3) x

Solving:

42/5 = 10x/3

Now, we can cross multiply again to solve for x:

42 3 = 5 10x

126 = 50x

Divide by 50:

x = 126/50
x = 2.52

Therefore, the solution is x = 2.52.