Finding the hypotenuse I have the question "The force vectors in the following diagrams are all coplanar but not drawn to scale. Use appropriate trigonometry to answer the following questions. 
Calculate the resultant force on the following objects and the acceleration it produces."

For this I have made a triangle and have used Pythagoras to find the length of the hypotenuse R. The answer is get for this is:

However, the solutions say that the answer for the length R should be 4.2 N.
I do not understand how This is achieved.

First combine two horizontal forces. As they are in opposite direction. So you got 3 N as resultant force. Now,
$\sqrt{(3)^2 + (3)^2} = \sqrt{18}$ = 4.24 N
