The two ships follow straight-line courses at an angle of 120 ° to each other: the first at a speed of 60 km / h

The two ships follow straight-line courses at an angle of 120 ° to each other: the first at a speed of 60 km / h, the second at 80 km / h. What is the speed of the first ship relative to the second?

Their relative speed will be the scalar product of their motion vectors, i.e.:
60 * 80 * cos120 = 60 * 80 * – (1/2) = -2400 km / h = -666.6 m / s
the relative speed is negative, because the figure shows they move away from each other
Based on this, we can conclude that the answer is – 666.6 m s