Matlab returns answer with another variable z. What does this mean?

gamma = 1.4;
U2b = 1000;
U2a = 800;
T2a = 300;
A2a = sqrt(gamma*R*T2a);

syms gamma Usb A2a U2a U2b
eqn = (((gamma+1)*((Usb/A2a)^2))/((gamma-1)*((Usb/A2a)^2)+2)) - ((Usb-U2a)/(Usb-U2b))==0;
SolUsb = solve(eqn,Usb)

Returns

ans =

root(A2a^2*U2a - A2a^2*z - (z^2*(U2a + U2b - U2a*gamma + U2b*gamma))/2 + z^3, z, 1)
root(A2a^2*U2a - A2a^2*z - (z^2*(U2a + U2b - U2a*gamma + U2b*gamma))/2 + z^3, z, 2)
root(A2a^2*U2a - A2a^2*z - (z^2*(U2a + U2b - U2a*gamma + U2b*gamma))/2 + z^3, z, 3)

Answer 1

[Torsten]

Use

SolUsb = solve(eqn, Usb, 'MaxDegree', 3)

instead.

Answer 2

[Star Strider]

Put the syms declaration first and use vpasolve —

syms gamma Usb A2a U2a U2b
R = 8;
gamma = 1.4;
U2b = 1000;
U2a = 800;
T2a = 300;
A2a = sqrt(gamma*R*T2a);


eqn = (((gamma+1)*((Usb/A2a)^2))/((gamma-1)*((Usb/A2a)^2)+2)) - ((Usb-U2a)/(Usb-U2b))==0;
SolUsb = vpasolve(eqn,Usb)

SolUsb = 

.