Solving equations for PWM Selective Harmonic Elimination

How can solve the simultaneous equations to get the angles for selective harmonic elimination in a PWM VSI.

For example, how can I solve these equations

m = (4/pi)*(1-2*cos(a) + 2*cos(b) - 2*cos(c))

0 = (1 - 2*cos(5*a) + 2*cos(5*b) - 2*cos(5*c))

0 = (1 - 2*cos(7*a) + 2*cos(7*b) - 2*cos(7*c))

I need to get the values of angles a, b and c.