Infuse Learning to any teacher that wanted to use a student feedback tool that supported drawings and mathematics symbols. It was great because students could draw responses to your questions and you would see their responses instantly in your Infuse Learning account. Unfortunately, earlier today Infuse Learning announced that they will be shutting down on April 3rd. While I'm not aware of anything quite as slick and robust as Infuse Learning, there are some feedback tools that might fill part of the void left by the closure of Infuse Learning.
Otus is a free online learning environment that designed for use on iPads and Chromebooks. One of the features of Otus is the option to create quiz questions in which students have to annotate a document or image with the drawing tools in the app. You could create a quiz question based on a document that has math problems typed on it. Your students could then write on the document to submit their answers to the problems. Click here to read more about all of the features of Otus.
MathChat is a free iPad app on which students can draw and write free-hand responses to math problems. The "chat" part of MathChat refers to the ability to collaborate and chat with other users of the app to get assistance with problems. Sabba Quidwai wrote a great post about using MathChat in the classroom. You can see Sabba's detailed post here.