The lengths of the rectangles are 7 and 12 respectively. Form the ratio 7/12. The widths of the rects. are x and 5 respectively. Form the ratio x/5. Now equate these two ratios:
7 x
--- = ---
12 5
Solve this for x. One way to do this would be to cross-multiply, obtaining 12x = 35, and solving this result for x. x will be a fraction. Write the numerator and denominator in the boxes given.