Atriangle has side lengths of 11cm and 9cm. which could be the value of the third side, 20cm or 15cm?

You would be looking to find the hypotenuse, being that both of the options are more than the values of the sides you gave
The equation is a^2+b^2=c^2
11^2+9^2=c^2
121+81=c^2
202=c^2
Now you need to find the square root of 202
So the exact answer would be √202
An approximate answer to the nearest hundredth would be 14.21