Answer:
x ≈ 14.7
Step-by-step explanation:
Assuming the triangle is right then use Pythagoras' identity.
The square on the hypotenuse is equal to the sum of the squares on the other 2 sides, that is
x² + 15² = 21²
x² + 225 = 441 ( subtract 225 from both sides )
x² = 216 ( take the square root of both sides )
x = ≈ 14.7 ( to the nearest tenth )
