that'd be m = E/c^2 (after dividing both sides by c^2).
without deriving the equation, you can still check whether the units are right:
basic units:
mass : kilograms (kg)
distance: meters (m)
time: seconds (s)
derived units:
speed = distance/time : m/s
acceleration = speed/time : m/s^2
energy = mass * acceleration * distance : kg*m^2/s^2
the last equation might not be familiar, but it's the expression for the energy of motion. if a mass is accelerated at a constant rate over a distance, multiplying all 3 values together tells you how much energy was added by the acceleration.
In the same way that you can only get an acceleration by dividing distance by time squared, I hope you can see, to get an energy from a mass and a speed, you have to multiply mass*speed^2. as it happens if you use the speed of light, the result is the energy inherent in a massive object when it's at rest. since in our everyday experience speeds are typically much less than that of light, that means that most of the energy in the stuff we see is present as mass, not motion. I hope that helps a bit... clear enough?
Thats why heavier shot strikes harder and travels further.