loes=NSolve[2 c^3/(c^2-1)==15,c];
a=c/.loes[[2]];
r1=2 a^3/(1+a^2)^2;
q1=2 a^3/(a^2-1);
g1[u_]=r1(1-u/q1);
b=c/.loes[[1]];
r2=2 b^3/(1+b^2)^2;
q2=2 b^3/(b^2-1);
g2[u_]=r2(1-u/q2);
h[u_]=u/(1+u^2);
q=Min[q1,q2];
abb=Plot[{g1[u],g2[u],h[u]},{u,0,q},ImageSize->{400,300}];