This is the concept of geometry, the third value of the triangle whose two sides are 8 cm and 15 cm can be obtained using the Pythagorean theorm:
c^2=a^2+b^2
where:
a=8 cm and b=15cm
thus substituting the above in our equation we get:
c^2=8^2+15^2
c^2=64+225
c^2=289
thus;
c=sqrt289
c=17
The third side could be 17 cm