I am having trouble solving the following problem.
Three people are pulling on a tree. The first person pulls with 15N at 65 degrees; the second with 16N at 135 degrees; the third with 11N at 195 degrees. What is the magnitude and direction of resultant force on the tree?

You need a sketch for those kinds of questions. After that, calculate the components of the forces, preferably vertical (at 0 degrees and 180 degrees) and horizontal (at 90 and 270 degrees).

I'll break the first force for you.

With the sketch, you'll find that the hypotenuse is of length15, and the angle with the vertical 65 degrees.

With the principles of cosine, cos 65 = Adjacent side/Hypotenuse

cos(65) = \frac{Adj}{15}

Adj = 15 cos(65)

The adjacent side is the vertical component.

Now, the opposite side:
From the principle of sine, sin 65 = Opposite side/Hypotenuse

sin(65) = \frac{Opp}{15}

Opp = 15 sin(65)

That is your horizontal component.

Now find the components of the other forces with the help of your sketch. Take care of the directions.

Then, find the resultant forces. Some vertical components will cancel themselves, as will do some horizontal components.

After, that, recombine the resultant forces with the Pythagoras' Theorem, and tangent principle to find the angle.

Post your answer.

Sorry its not understandable to me

What do you not understand? Post what you have done up to now.