To find the measure of x, we can use Pythagoras' Theorem of c² - b² = a². In this case, c would be 26 (AB), and b would be 22 (AC), so you can plug in the numbers and write it as
26² - 22² = x²
676 - 484 = x²
192 = x²
√
x = 13.86 (2 d.p)
So your answer is x = 13.86, I hope this helps!