Hey guys, So I've actually learnt a fair bit about trig identities the last few weeks and beginning to understand how they actually work thanks to Irrational, Mute and some prompting from Hurkyl. I'm still having trouble with transposing which I think should be fairly simple. The equation is solving steering geometry with two points of rotation that locates two points in space. The equation which works out the length between two points is: D = | P1 - P2 | with | | denoting the length of the resulting 3D vector. I know D (the length) and I also know P2 (the x,y,z position of point 2). P1 is what I'm having trouble with. The equation for P1 is: P1 = Pf + (i1f * Rf * cos(Vf)) + (i2f * Rf * sin(Vf)) I know Pf (the x,y,z position of the axis of rotation), i1f (the x projection vector of P1's rotation axis), Rf(the length between P1 and the axis of rotation) and i2f(the y projection vector of P1's rotation axis). From those I know all but Vf, which is an angle from P1 to the x axis and P1, which is solvable by substituting it into the 1st equation D = | P1 - P2 |. Vf is where I'm having the problem. How can I transpose it to make it the subject. I've tried using the identity R * cos(x - alpha) but still have the same problem of having 2 instances of Vf in the equation as in P1.x, P1.y and P1.z will all use different values for R and alpha. I hope this makes sense. If not I'll post up the images from the white paper I'm using. Any help is much appreciated.