Answer:
15
Explanation:
Since this is a right angled triangle we can use pythagoras theorem to work out the length of the hypotenuse, or the the longest side.
pythagoras theorem says that a²+b²=c² with c² being the hypotenuse, or missing side here.
12²+9²=225
Because 225 is equal to C², or C squared, we must take the square root in order to find the actual length.
√225 = 15