Find the length of the diagonal of a rectangle whose length and width are 35cm and 12cm respectively?

Easiest thing to do here would be to draw a diagram and label the sides with their lengths and draw a line for the diagonal. You can see the diagonal forms part of a right-angled triangle, with its shorter lengths 35 cm and 12 cm.

So using Pythagoras's Theorem, x2 = 352 + 122 = 1369

x = square root of 1369 = 37 cm.