okay so a triangle has 3 sides. right now you’re given the lengths of 2 of them. to find the 3rd length you have to put it into an equation which is a2+b2=c2. side C is the hypotenuse (the longest side on a triangle). since, you only have side A and C you need to use c2-a2=b2. in your case it would be 12^2-6^2=c2. 144-36=108. to find the actual length of the side you need to find the square root of 108 which is 6/3 (/ is the square root symbol).
if you need a deeper explanation or more help just message me :))
