His warnings are warning him that converting double to float will potentionally lose information. This is because doubles have a greater accuracy than floats, so converting double to float will lose accuracy.
Number literals like 2.0 or 4.523 are by default doubles. To make float literals, simply append "f", like 3.45f.
Also, I remember reading that the PI macro in header files are implementation dependent, as in the names aren't consistent from compiler to compiler. Making your own PI onstant would be better.
I don't remember in C++ how to access the predefined version of PI exactly. I could do it for you in C, but that's not the point, is it? As long as you get the right answer from the program, you should be able to safely ignore those two warnings, but only so long as you understand that somewhere there's an identifier with the same name declared as a double, which just means that it's more precise. Since you're only using about 1/3 of the precision of a float, you shouldn't be having too much of an issue (which is why I say that you can ignore them). However, if you want to do anything on a grander scale than just turning in a homework assignment, you'll need to look up how PI is defined in the C++ libraries.