adiabatic walls boundary conditions

Hello LBM friends;

i am using this scheme(the simple extrapolation scheme) to implement boundary conditions on adiabatic walls:

g+k(I,jmax)=(4* g+k(I,jmax-1)- g+k(I,jmax-2))/3

could you telle me please how to write this on matlab?!i mean the sign plus(+) ? where g is the distribution function for temperature’
Thank you a lot ,
Have a nice day LBM Friends

g[sup]+[/sup][sub]k/sub=(4* gg[sup]+[/sup][sub]k/sub- gg[sup]+[/sup][sub]k/sub)/3

g[sup]+[/sup][sub]k/sub=(4* g[sup]+[/sup][sub]k/sub- g[sup]+[/sup][sub]k/sub)/3