Back EMF Qn: Help Needed

[dermite]

I would like some help w/ the below MC Qn
How do you approach a qn like this?
Which formula should be used?

Thanks

[Fizzycyst]

EMFnet = EMFsupply - EMFback

So, you are told that the motor delivers 4A and has a resistance of 3.5 Ohms.

Using Ohms Law, you can determine the voltage across the coil (EMFnet) as 4 x 3.5 = 14V

Using the equation above, you end up with:

14 = 240 - EMFback

Therefore EMFback = 226V, answer is C.

I would like some help w/ the below MC Qn
How do you approach a qn like this?
Which formula should be used?

Thanks