Question 970160
The isosceles triangle can have the base split into two halves, each 7.5 cm long.

Now the altitude is one of the legs of a right triangle with the other leg 7.5 and hypotenuse 21

56.25 (1/2 of base squared) + x^2 = 441 (21^2)   Pythagorean Theorem.
Subtract 56.25 from each side.
x^2= 384.75

x=19.61 cm  positive root only.

That is the altitude.

The area is (1/2) bh
base is 15 cm, altitude 19.61 cm

0.5 * 15 *19.61=147.075 cm^2