You need the kinematic equation below:
[tex]v2^2 = v1^2 + 2a * d
[/tex]
re-arrange to get the velocities on one side:
[tex]v2^2 - v1^2 = 2a * d[/tex]
Then simplify things to make it easier to see. Since v2 = 0 because the whale is not moving once it reaches 7 meters, and we need to find v1 from v1^2, we can write the equation like this:
[tex]v1 = sqrt(2a * d)[/tex] Where a = acceleration due to gravity = 9.8m/s^{2}
[tex]v1 = sqrt(2 * 9.8 * 7)
= 11.7m/s[/tex]
