Answer:
+4√5
Step-by-step explanation:
Note that this is a right triangle. Therefore, the lengths of the three sides are related through the Pythagorean Theorem: a^2 + b^2 = c^2, where c is the length of the hypotenuse.
Thus, x^2 + 19^2 = 21^2, or, after solving this for x^2,
x^2 = 441 - 361 = 80
x is therefore +√80, or +4√5.
