Plot3D showing nothing


I tried to plot f(x,y) = cos(x + y) + sin a(x-y) where 0<=a<=10. I used Plot3D[f[x, y], {x, 0, 10}, {y, 0, 10}, AxesLabel -> {"x", "y"},ColorFunction -> GrayLevel]and it’s not working and I think it’s because I left out the condition for ‘a’. I am also unsure how to include ‘a’ inside the Plot3D function.