Answer
$T=10.5~N$
Work Step by Step
Find the volume of the rock:
$V={m \over \rho_r}={28~N \over (9.8~m/s^2)(1200~kg/m^3)}=2.38\times10^{-3}~m^3$
Find the Buoyant force when the rock is submerged:
$B=\rho g V=(750~kg/m^3)(9.8~m/s^2)(2.38\times10^{-3}~m^3)=17.5~N$
Sum all the forces to find the Tension in the string:
$\sum F=T+B-W=0$
$T+17.5~N-28~N=0$
$T=10.5~N$
