Hi

I am really stuck on this problem. It reads as such:

A uniform triangular lamina is isocceles and has the line y= 4 as its axis of symmetry. One of the vertices of the triangle is the point (2,1). Given that the x-co-ordinate of the centre of mass of the lamina is -3, find the co-ordinates of the other two vertices.

I am not sure where to begin. Any hints or guides please?