LDR  R0, R6, #3   ;  load the value of x
ADD  R1, R6, #4   ;  put the base address of grid into R1
ADD  R1, R0, R1   ;  calculate address of grid[x]
LDR  R2, R1, #0   ;  grid[x]
ADD  R2, R2, #2   ;  grid[x] + 2
   
LDR  R0, R6, #3   ;  load the value of x
ADD  R0, R0, #1   ;  x+1
ADD  R1, R6, #4   ;  put the base address of grid into R1 
ADD  R1, R0, R1   ;  calculate address of grid[x+1]
STR  R2, R1, #0   ;  grid[x+1] = grid[x] + 2;