The following error occurred converting from sym to double:
Error using mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead.

Error in Untitled (line 7)
s(2)=y(2)-c-a*y(1)-b*s(1);


是什么问题，请大家帮帮忙看看。