how to create this matrix?

I think it would look something like this...

iLength = 5; %rows
jLength = 7; %cols

b=zeros(iLength, jLength);

for i = 1:iLength
    for j = 1:jLength
        if i == j; b(i,j) = 1; end
        if i < j; b(i,j) = (-2)^(j-i); end
    end
end

disp(b)
            

gives a result of:

     1    -2     4    -8    16   -32    64
     0     1    -2     4    -8    16   -32
     0     0     1    -2     4    -8    16
     0     0     0     1    -2     4    -8
     0     0     0     0     1    -2     4