What is the length of missing side b in the triangle 7cm and 13 cm?

It might be pythagoras therom but it can only be Pythagoras when the traingle has a right angle. If it does then try to work it out using phythagoras.

If the angle between the given sides is B, then:

b2 = a2 + c2 - 2ac cos B

⇒ b2 = (7 cm)2 + (13 cm)2 - 2 x 7 cm x 13 cm x cos B

⇒ b = √(218 - 182 cos B) cm

If it is a right angle triangle, with B the right angle, cos B = cos 90o = 0 and this becomes Pythagoras making the side:

b = √218 cm

≈ 14.76 cm

If there is a right angle, not between the 7 cm and 13 cm, then the 13cm side is the hypotenuse (as the hypotenuse must be the longest side) and the other side is:

b = √(132 - 72) cm

= √120 cm

≈ 10.95 cm