The rectangle they give you is 12 units tall. This is the result of scaling by a factor of 1.5, ie it is 1.5 times taller than what the answer will be. Let the answer rectangle have a height of h. This means h*1.5 = 12, or 1.5h = 12
Divide both sides by 1.5 to find that h = 8. The answer rectangle's height is 8 units.
The width's will be treated in a similar manner. I picked on the height since it's easier to see that the height lines up with 12 (the width seems to be between 2 and 4, but its not clear if its at the midpoint).