So basically the side length went from 3 to 7.5, so we have to find the factor which caused the increase. We have to find what times 3 makes 7.5
So our equation is 3×x=7.5
From here on it's just algebra, we'll divide each side by 3 to isolate x
7.5/3=2.5
x=2.5
So the scale factor applied to the first rectangle is 2.5