Based on an assumption you're using z charts for this (and I could be wrong), you need to get the z scores for the 300 and 400 first.
z = (x - μ) / σ
A z score is saying how many standard deviations it is.
So for instance, taking a point 300 - mean of 500 = -200. That's the distance from the mean. Then dividing by 90 tells you how many standard deviations that is.
You'll want to do that for both points, 300 and 400.
Both of those are to the left of the mean, and you need that "slice" between the 2. That can be the trickiest to do, and unfortunately the hardest to explain, especially without pictures.
z charts can vary, so I don't even know what your specific charts would look like. However, the slice between those 2 points is one point to some location less the other point to the same location.
For instance, if you have a tail chart, the distance from the 400 to the end of the left tail gives you that whole entire section. So then you want the area from the 300 to the end of the left tail
subtracted off. (Draw it and think about that a minute.)
The area to the 400 minus the area to the 300 leaves you with the area in between, the slice.
You can also do this if you have a chart that shows from the 0 out to a point, and you can even do it if you have a chart that goes from one end and cross over the center. Either way, it's the difference between the two. (Of course, you will be using the z scores for those on the charts.)