I believe the formula to find the height would be h= 2a/b and if you use that formula and plug in the area (21.7) of each triangle as well as the base (6) then you would get a height of 7.2 cm. I hope this is correct! :)
Coplaner is a term used in geometry referring as points which rest on the very same plane; typically orbiting the same way. The point can be rested as 3 or several points in the same plane.