very well xplained thnk u so muchOk so.... firstly, you need to look at the directions the forces are acting. BOTH forces are acting inwards onto the particle, as a result of this, the resultant force will also act inwards.
The next step is figuring out the direction in which it acts, you can find this out by attaching the tail of one force to the head of the other.
View attachment 51828
Then you use your knowledge of vector addition and you would end up with something like this...
View attachment 51829
and then you move towards something like this,
View attachment 51830
Now for the second part, to prove that the resultant force is 6.6 N. You use the diagram in which you applied the nose to tail method.
View attachment 51831
From this you can use the cosine rule, since you have a) two known sides and b) one unknown side with a known angle
R^2 = (2.5)^2 + (7.5)^2 - ( 2 x 7.5 x 2.5 x Cos 60 )
R^2 = 62.5 - 18.75
R^2 = 43.75
R= 6.6143 N
Which rounds up to 6.6 N, since both 2.5N and 7.5N are given to 2 significant figures.
Hope that helped!