Answer: 9
====================================================
Step-by-step explanation:
The slanted left-most sides of the figures are 24 and 8, when moving from left to right.
Let's reverse the direction and move from right to left. The jump from 8 to 24 is "times 3", which is the scale factor when moving from the smaller figure to the larger figure.
The smaller figure has 3 up top. Apply the "times 3" scale factor to get 3*3 = 9 as the top side length of the larger figure.
In other words, every side of the smaller figure is tripled to get the larger figure.
----------------
Here's another way to solve.
24/8 = x/3
24*3 = 8*x .... cross multiplying
72 = 8x
8x = 72
x = 72/8
x = 9 is the final answer.