Answer:
20
Step-by-step explanation:
We can use the Pythagorean theorem since this is a right triangle
a^2+b^2 = c^2 where a and b are the legs and c is the hypotenuse
21^2 + b^2 = 29^2
441+b^2 =841
Subtract 441 from each side
441 +b^2 -441 = 841-441
b^2 = 400
Take the square root of each side
sqrt(b^2) = sqrt(400)
b =20
