Jesutofunmi - look closely at your code
for k = 1:n-1
Lsum = a +dx*(3*(k)^2+4);
end
display(Lsum)
On each iteration of the for loop, you are ignoring what came previously so you end up with Lsum being assigned the a value from the last iteration of your for loop. You need to sum this value with what came previously as
Lsum = 0;
for k = 1:n-1
Lsum = Lsum + a +dx*(3*(k)^2+4);
end
display(Lsum)
This still won't give you the correct answer though. What is the function (curve) that you are calculating the area for? Define it as an anonymous function like
f = @(x)3*x^2 + 4;
The above is an example based upon what you have shown but it may not be the function that you are interested in. Then your sum would become
Lsum = Lsum + dx*f(k);
where k is an input. So what should that input be? Since this is homework, we can only give out hints. Review your class notes and try to figure out what it should be. See http://mathinsight.org/calculating_area_under_curve_riemann_sums for guidance as well.
