Description for Correct answer:
\( \Large \left(a-b\right)^{2} = a^{2}+b^{2}-2ab \)

=\( \Large 24 - 2 \times 4 = 24 - 8 =16 \)

Therefore, \( \Large \left(a-b\right)^{2} = 16 \)

\( \Large a-b = \sqrt{16} = 4 \)