High school Physics!! What is the Mach number for a shuttle if it starts re-entry...?

What is the Mach number for a shuttle if it starts re-entry after traveling around earth in 1.495h at an orbital radius of 6.73 x 10^6 m? ume that the air temperature is -30 degrees C( Celsius). I know the asnwers are 7.86x103m/s and the Mach is 25 but I have no idea how to get it. Please help! Thank you!!