I'm either using the wrong formulas or using the right formula in the wrong way...

Why don't you

*tell* us what formula you're using, so we can see where you may be going wrong.

Likewise show us your working on the slope of the graph. It looks about right to me.

On a more general point, you are assuming that "Rate = k[A]" means d[A]/dt = -k[A], in which case your answers to the first part are correct. However, "rate of reaction", unless more specifically defined, usually means 1/ν

_{i}*d[R

_{i}]/dt, where ν

_{i} is the stoichiometric coefficient of reagent R

_{i} (positive for products, negative for reactants). Thus for your reaction as written

Rate = -1/2*d[A]/dt = -1/4*d[ B]/dt = d[C]/dt = 1/3*d[D]/dt

In this case [A]/[A]

_{0} = e

^{-2kt}The problem with this is that before 1.2575 min all the B (the limiting reagent) is gone, so the question can't be answered. (Unless that is

*supposed* to be the answer, but I doubt it.)