Typically if you're dealing with a small residential income property (2-4 units), you would use the Gross Rent Multiplier (GRM) to determine overall value of a property based on gross market rents of similar units in the area. Generally in Los Angeles city, becasue of rent control restrictions, or high percentages of owner occupied units, the GRM method is given secondary weight to the sales comparison approach in determining market value, even for 2-4 unit properties.

Put simply, the cap rate is the net operating income divided by the sales price. It is not possible to calculate the cap without all the information.
Dorene Slavitz
Residential & Commercial Realty