50g

I am trying to solve three simultaneous equations and I can't seem to 
figure out how to do it.   I've gone through the manual and keep 
getting an error trying to use msolv

I have 
M1=p*0^2+q*0+r
M2=p*(L/2)^2+q*(L/2)+r
M3=p*L^2+q*L+r

The answers are p=-3 q=102 and r=-578

Can anyone give me the syntax to get this from a 50g?

I've also tried entering them as vectors and then using solve but it 
only gave me the trivial solution (0 0 0)

Thank you.