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:
\(exponential\ form = x^{1/3}\\radical form = \sqrt[3]{x}\)
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 \(\sqrt[3]{a}\) to \(a^{1/3}\). Then, because the a is the same after the 2 and the 3, you can add them up to get \(5 a^{1/3}\).
For #24, you'd do almost the same thing (change the radical-b's to \(b^{1/4}\) and \(b^{1/3}\). However, you can't add them up because the exponents are different (1/4 and 1/3), so you'd leave it as \(3b^{1/4} - b^{1/3}\)
