Answer
$F=18.3N$
Work Step by Step
We know that
$a=\frac{v^2-v_{\circ}^2}{2\Delta x}$
We plug in the known values to obtain:
$a=\frac{(0m/s)^2-(1.25m/s)^2}{2(2.2m)}$
$a=-0.355m/s^2$
Now $F=ma$
We plug in the known values to obtain:
$F=(51.5Kg)(-0.355m/s^2)$
$\implies F=-18.3N$
The negative sign shows that the force is opposite to the direction of motion of the swimmer.