Finding value of slip in an 3-phase delta connected induction motor
\$N_R\$ is your rotor rotational speed represented in revolutions per minute (rpm). In the question you've been given a rotational speed in revolutions per second. Should be a easy as:

$$ N_R \text{ [rpm]} = 16.2 \text{ [rev/s]} * 60 \text{ [s/min]} = 972 \text{ [rpm]} $$

Also, the equation for synchronous rotational speed, \$N_S\$ is:

$$N_S \text{ [rpm]} = \frac{120f \text{ [Hz]}}{p \text{ [poles per phase]}} = \frac{120 * 50}{6} = 1000 \text{ [rpm]} $$

Where \$p\$, your poles per phase, will be eighteen divided by three, for a three-phase motor.