I just whipped this up for you:picture
block 1 explains the angles
block 3 explains the the different magnitudes of all the gravity forces (Fz is dissected into Fz perpendicular and Fz parallel)
block 4 explains how much torque 1 motor should have
block 5 explains all the symbols
block 2 explains how you should calculate the same for multiple engines
Keep in mind that the torque should also conquer air friction and friction between the tire and the plane the robot is driving on, so you might need it a little higher.
The final formula:
MotorForce > ((sin(angle) + (M * 9.81)) / WheelRadius) / NumMotors
Does that explain enough?