we know that
Applying the Pythagoras Theorem
c²=a²+b²
in a square
a=b
c is the diagonal-----> 84 m
c²=2a²--------> a²=c²/2-----> a²=84²/2-----> a²=3528-----------> a=√3528 m
the answer part a) is
√3528 m or 59.40 m
the area of the pen is a²------> 3528 m²
the answer Part b) is
3528 m²
