System of equations error

Why do I recvie an error saying: Undefined function or variable 'z'

when trying to solve the following system of equations:

sol=solve(['5=x','-6x+10y-3z=0','-y+51z=0'],[x,y,z])

Answer 1

[Kevin Phung]

syms x y z
sol=solve([x==5 ,-6*x+10*y-3*z == 0,-y+51*z==0],[x y z])