Can 26x^2 be written as x*sqrt26?

no,

26x^2 ≠ x√26

Whatever made you think so.

let x = 4
26(4^2) = 26*16 = 416
xV26 = 4√26 = appr 20.4

is 416 = 20.4 ????

However, you could write 26x^ as (x√26)^2