Question 279400
Not hypertense but hypotenuse
A triangle has three sides. We know two of them.
How do we find the third side of a right triangle?
a^2+b^2+c^2
where c is the hypotenuse and a and b are the legs.
a+b+c=perimeter
so first we need to find the missing side.
a^2+9^2=15^2
a^2+81=225
a^2=144
a=12

We could also  have deduced the third side since this is a multiple of the classic triplet right triangle 3, 4,5
3*3=9
3*4=12
3*5=15
So now you have the three sides.
You still need to add them together to get the perimeter

a+b+c=perimeter