For an uniformly accelerated motion, we can write
[tex]2ah=v_f^2-v_0^2[/tex]
where [tex]a=g=-9.81 m/s^2[/tex] is the acceleration of this motion, which in this problem is the gravitational acceleration, with a negative sign because it points downward, against the direction of the motion; h=0.540 m is the distance covered by the flea, and [tex]v_0[/tex] is the initial velocity.
At the maximum height, the velocity is zero, so [tex]v_f =0[/tex]. Therefore we can solve to find [tex]v_0[/tex]:
[tex]v_0 = \sqrt{2ah}= \sqrt{2(9.81 m/s^2)(0.540 m)} =3.25 m/s[/tex]
