Okay, so this is a right- angled triangle so you can use pythagoras’ theorem.
A^2 + B^2 = C^2
Where c is the hypotenuse ( the longest line opposite the right angle).
So in this question we know
B= 11 and C= 24
So If we rearrange we get
C^2 - b^2 = a^2
So
24^2 = 576 and 11^2 is 121
576-121 = 455
So a^2 is 455 so we need to square root that to find the missing leg which is
21.330729
Then we need to round it so we get
21.3
