You need to use the Pythagorean theorem starting with the triangle on the far left and progressively moving to the right. (Comment if you want further explanation)
First you use Pythagorean theorem in solving the triangle closest to the bottom. You would get the square root of two. Then you use Pythagorean theorem again on the second triangle and receive the answer of the square root of three you do the Pythagorean theorem again and receive the answer of the square root of four. Simplify that and get 2 cm.
