The problem is the way you index your for loop:
for m=6:3
The colon (link) operator default behaviour is to increment by 1 from the first value to the last. Here, the last value is lower than the first, so the loop limit is already satisfied at the start, and the loop does not execute and so does not calculate ‘a’ (or ‘b’). To modify that behaviour, set the increment to -1:
for m=6:-1:3
a(m)=m^2
b(m)=m*s
end
The loop now executes, although since ‘s’ is not defined here, will crash on the ‘b’ assignment line.
