Write 5.26 as a fraction, then simplify the square root of that fraction.
5.26 = 526/100, so
sqrt5.26 = sqrt(526/100)=(sqrt526)/sqrt100=sqrt526/10
sqrt 526 cannot be simplified.
(526 = 2*263 and is not divisible by perfect squares: 4, 9, 16, 25, and that's enough to check, because 25^2 = 625 > 526)