Plane Speed: Calculating Mach with Shockwave Delay

[DmX0rZ]

Homework Statement

A plane passes from 900 meters over my head. I hear the shockwave after 2.2 seconds. At what speed in Mach is the plane flying ? Speed of sound = 331. 4 m/s

Homework Equations

Machs = speed of plane / speed of sound

The Attempt at a Solution

I don't have any clue where to go with this, maybe find the angle of the cone ? it gave me an angle of like .14 degrees and an impossible answer of the plane speed... dah

Thanks in advance

 

[Dick]

Don't double post.