Answer
$5.3cm$
Work Step by Step
The maximum height attained by a flea can be determined as
$K.E_A+U_A=K.E_B+U_B$
$0+(2\times \frac{1}{2}Kx^2+0)=0+(0+mgy_B)$
This simplifies to:
$y_B=\frac{Kx^2}{mg}$
We plug in the known values to obtain:
$y_B=\frac{(26)(1.0\times 10^{-4})}{(0.50\times 10^{-6})(9.81)}$
$y_B=0.053m=5.3cm$