Answer:
40
Step-by-step explanation: A rectangle is a quadralaterial with 4 90 degree angles made up of two diagonals. If we draw one diagonal, it would split the rectangles into two right triangles. Since a rectangle has a right angle, we can use pythagorean theorem to solve the missing side.
where a and b are the length and widith and c is the diagonal length. Let plug it in

900+
=2500

40
b=40