You did not give any other clues so logically it is not possible to find the other length unless an angle is given. So if we think that the triangle is a right angle triangle then we can use the pythagorean theorem formula to calculate the other side which is:
c^2=a^2 + b^2
Now we know can say that one side 9 cm is,"a" and 12 cm is, "b" Now place the formula values accordinly to find, "c"
c^2= 9^2 + 12^2
c= square root of 225
c= 15
The exact value for the third side can be 15cm. But without any other hints it is not possible to find the other side length of the triangle.
