I need help ASAP! Pease help me understand this please and thank you

Answer: the missing side lenght is: 75ft
Step-by-step explanation:
Phytagoras theorem: 21^2+72^2=x^2 / x is the missing side lenght
[tex]x=\sqrt[2]{21^{2} +72^{2} }[/tex]
[tex]x=75ft[/tex]