Would I be right if I said 21pi/10 and - pi/10 are two angles coterminal with pi/10?

Two angles are coterminal if they differentiate in a factor of k * (2pi). This is a multiple of 360 degrees.

In this case 21 [pi / 10] = [pi / 10] + 20 [pi/10] = [pi/10] + 2pi, then 21pi/10 is coterminal with pi/10.

In the case of - pi/10 you cannot get to pi/10 by adding a multiple of 2pi, then they are not coternminals.

Conclusion: you would not be right.