torque is defined as the perpendicular component of a force times the length of the lever arm. It measures the how much a body (in this case the door) will be motivated to rotate.
The formula for torques is τ = Frsinθ where θ is the angle between the lever arm, r, and the force. If the force is exerted perpendicular to the lever arm, the formula is τ = Fr with units of N-m (These are not the same as Joules which you may learn as you go deeper into rotational mechanics)
In this case the F is perpendicular and we are interested in the torque that will make the door rotate on its hinge: τ = (10,4N)1.25 m)