How to find the third side of a triangle if the other two are known? if we know that one is 7cm and the other is 16cm.

Let’s assume that this triangle is rectangular. The Pythagorean theorem can be applied to this type of triangle. The essence of this theorem is that the sum of the squares of the legs is equal to the square of the hypotenuse.
Imagine that the two known sides of a triangle are legs. Then we find the hypotenuse:
a² + b² = c²
7² + 16² = 49 + 256 = 305
c = √305 = 17.5 cm.
Answer: 17.5 cm.