Try
[a,b,kc] = solve(x-y,w-z,t-u,a,b,kc)
Your question about GUIDE is not clear. You can get() strings from edit boxes and str2double() the strings.
Are you trying to put the equations themselves in the edit box? That is possible: sym() the string and pass that into solve()
eq1 = sym(get(handles.BOX1, 'String'));