In an isosceles trapezoid, the base is 4cm and the height of the trapezoid is 14cm. Find the side of the trapezoid.

Draw the height from the point of intersection of the small base with the left side. This height forms a triangle. Where the height and part of the base are the legs, and the side is the hypotenuse.
Find a piece of a larger base. Since the trapezoid is isosceles, then
(14-4) / 2 = 10/2 = 5.
By the Pythagorean theorem:
Side = √ (5 ^ 2 + 6 ^ 2) = √ (25 + 36) = √ (51)