Puzzle ZBSB 

Workings

Hint
Remember that 3 raised to the 5th power is 3 x 3 x 3 x 3 x 3.

Answer
3 raised to 5th power.

Reasoning
3 raised to the 5th power = 3 ^ 5 = 3 x 3 x 3 x 3 x 3 = 243

and

5 raised to the third power = 5 ^ 3 = 5 x 5 x 5 = 125.