Before answering the exact question you sent, you have to understand one simple principle: there are two forms of a variable with a fraction as an exponent, exponential form and radical form.
Example:
[tex]exponential\ form = x^{1/3}\\radical form = \sqrt[3]{x} [/tex]
If the exponent is a fraction, the denominator of the fraction is the number on the outside of the radical sign.
So, for #23, you would change [tex] \sqrt[3]{a} [/tex] to [tex] a^{1/3} [/tex]. Then, because the a is the same after the 2 and the 3, you can add them up to get [tex]5 a^{1/3} [/tex].
For #24, you'd do almost the same thing (change the radical-b's to [tex] b^{1/4} [/tex] and [tex] b^{1/3} [/tex]. However, you can't add them up because the exponents are different (1/4 and 1/3), so you'd leave it as [tex]3b^{1/4} - b^{1/3}[/tex]
