I assume the given expression is
[tex]\sqrt x \sqrt[4]{x}[/tex]
We can write each root as a rational power of x, then add the exponents:
[tex]\sqrt x \sqrt[4]{x} = x^{\frac12} x^{\frac14} = x^{\frac12 + \frac14} = x^{\frac34}[/tex]
Then putting it back into radical form, we have
[tex]\sqrt x \sqrt[4]{x} = \sqrt[4]{x^3}[/tex]
