Answer:
=> The third side of the triangle is √53 mm
Explanation:
We know that this is a  right angled triangle , 
so , by using Pythagoras theorem  we can find out the third side of the triangle , 
=> P²+B²= H²
=> 7²+2² = H²
=> 49+4 = H²
=> 53 = H²
=> √53 = H 
The third side of the triangle is √53mm