That code works in the form you posted.
syms q0
Vy=[300 500 300]
q(1,:)=[q0-(Vy.*2.*4./8)]
The most common mistake for something like that would have been to initialize q using zeros(), such as
q = zeros(5,3);
If you had initialized q to double precision, then the assigning into q(1,:) would fail because what you are assigning needs to contain a symbolic variable.
The cure for that would be to use something like
q = zeros(5, 3, 'sym');
for new enough versions of MATLAB, or
q = sym(zeros(5,3));
if your MATLAB is older than that.
