Give the approximate bond angle for a molecule with a trigonal planar shape.

Trigonal means it has 3 "legs".

Because it is planar, the sum of the angles between all the legs must be 360°. Because the legs are all equally distanced from eachother, you divide the area uniformly.

This means you divide the 360° by the amount of legs:

360°/3 = 120°

Note: this only works in 2D planar configurations