$3\mathrm{.}4$ A $300-N$ force is applied at $A$ as shown. Determine $(a)$ the moment of the $300-N$ force about $D,($b$)$ the smallest force applied at $B$ that creates the same moment about $D.$please explain in details for part a why we multiply the the force in x by the vertival distance $200$mm and the force of y by $100$mm$.$ I thought we had to do the opposite. if you can maybe draw also a visual explaining this.