The only output from that code will be where you define a at the start of the program (without a semicolon at the end).
Your code is calculating correctly, but it's not displaying the final value of a.
You need to add a line after the loops to print the final value of a:
a = 0;
for i = 1:0.5:2
for j = 1:1:3
a = a + j;
end
end
disp(a)
