I've done part a, but I'm kind of confused about part b. How do I go about solving it? Is there another special formula for it?

 

In part b you are simply forcing the y-intercept to be zero. This is used in situation where you know that this must be the case.

 

Where did you get the formulas you used for part a)? Doesn't that source give the formula for the zero intercept case?

A google search for "Simple linear regression with zero intercept" finds: