Excel formula help

Hello and Thank you in advance for any assistance! My name is Tobey, and I working on a spreadsheet in Excel. I am trying to work o a spreadsheet, but I am having trouble with one of the columns. I need to get the percentage change in patients seen between two months. For example, new patients seen in January were 150, and new patients for February were 90. What formula would I use to find the percentage change between the two months?