The answer should be #15625x^12#, if the problem is written correctly.
Explanation:
#(25x^4)^3# is not a radical expression, but rather a simple problem involving powers.
To solve, simply apply the third power to the 25 and #x^4#, noting that when a variable is raised to one power and and then another, the powers are multiplied. Since it is #(x^4)^3#, you multiply 4 by 3 to get the power 12.