${\left( \frac{ 4{a}^{3}{b}^{3} }{ 8 } \right)}^{2}$
Cancel out the common factor $4$${\left( \frac{ {a}^{3}{b}^{3} }{ 2 } \right)}^{2}$
To raise a fraction to a power, raise the numerator and denominator to that power$\frac{ {a}^{6}{b}^{6} }{ 4 }$