syms all c1 Q L r x c2 c3 c4 Q0
E=((c1+c2*(((Q-Q0)/r+Q/r)/2)*Q0-(Q/r-x-(Q-L)/r)^2*r/2)+(Q-Q0)^2/2/r*c3+((Q-L)/r+x-(Q-r)/r)^2*r/2*c4)/((Q-L)/r+x)
A=subs(E,r,20)
B=subs(A,c1,10)
C=subs(B,c2,0.06)
D=subs(C,c3,0.08)
F=subs(D,c4,1.25)
G=subs(F,Q0,20)
H=subs(G,Q,40)
I=subs(H,x,6)
solve(I)