The ~ symbol means that the 2 shapes are similar, meaning the same shape but not the same size.
To determine the measure of side length EF, you must find the relationship between the 2 side lengths.
You should set these up as ratios of the length to the width. So, for ABCD, it is 30/45. Make this equal to x/75. If you simplify the first ratio to 2:3, you can then multiply by both numerator and denominator by 25 to get x = 25 in/ 75 in.
Another way is to use cross products with you proportion. 75 x 30 = 45x. Solve for x to get 50 inches.