This is the formula for calculating the scaler value, in case of inclined forces we find the angle between them too but the scaler value is found by this formula only. If you're getting into the mathematical aspect, it's the Pythagorus Theorem.
And what is "partly in the same direction? " Dude, a tenth standard question this is. There is absolutely no angle given bw the forces, I told the simplest thing possible without messing up the mind of a tenth grade student.

The exact scenario would be a force of 140N opposite to the direction of 40N. But is there an option for that? No. .

And besides, the OP has marked the answer as 150N. I got to the answer assuming the answer was 150N as given in the picture attached.