First create your first combo box (#1) linked to the table with the business types. In the properties for the combo box there is a parameter called "name" under the "other" tab. Make sure to remember this name or give it a name you will remember. Depending on how you created it Access may have given it a default name of the field from the table it is linked to. I hate this and usually rename it to something else.
Now create your second combo box (#2). Remember this name too. For #2 the row source will be the table that has the info you want. This table should also have the business type. Instead of just linking it to this table you will want to click on the ... button next to that column and get to the query builder. Pick the field with the info you want to show and the field with the business category. Then under criteria for the business category put in without the quotes "[forms].[NameofForm].[Name Of #1]".
Now go back to the properties of #1. Under the event tab, find the parameter "on change". Click on the ... to the right of this. You will get a windows that asks what you want to do. Pick code builder. You will be taken to the VBA screen. Where the cursor ends up type in the following.
Forms.NameOfForm.NameOf#2.Requery
Go save everything and see if it works.