pythagorean theorem
for a right triangle with legs (2 sides next to the the right angle) length a and b, and hyptonuse (longest side oposite right angle) is c

we can see that the legs are 3cm and 7cm
so a=3 and b=7 (you can do a=7 and b=3 but it doesn't matter)




sqrare root both sides


answer is 4th option