use the Pythagorean theorem, it is given that the shape is a rectangle so all opposite sides are congruent and all angles are right, since we know that the diagonal forms a right triangle we can use the pythagorean theorem
the Pythagorean theorm is a^2+b^2=c^2 where a and b are the legs (in this case the side of the triangle) and c is the hypotenuse which is the longest side in a right triangle
17^2+12.75^2=x^2
x^2=7225/16
now take the square root of both sides and you get -21.25 and 21.25
distance CAN NOT be negative so -21.25 isn't an option
