This question requires you to use Pythagoras' Theorem: c^2 = a^2 + b^2, where c is the length of the diagonal and a and b are the width and length of the rectangle
c^2 = 7^2 + 10^2
c^2 = 49 + 100
c^2 = 149
c = sq.root of 149 (as an exact answer) or 12.207 cm (to three decimal places)