We are given an object sliding down an incline with a friction force. A free-body diagram of the system is the following:
Now we will add the forces in the x-direction:

We will consider the forces that are in the direction of the movement as positive and the ones in the direction against the movement as negative. Plugging in the forces:

Now we solve for the friction force:

Multiplying both sides by -1:

Now we determine the x-component of the weight using the following triangle:
From the triangle we can use the function cosine as follows:

From this we can solve for the x-component of the weight by multiplying both sides by "mg":

Now we use this expression and replace it in the sum of forces:

Now we plug in the known values:

Solving the operations:

Therefore, the friction force is 171.5 Newtons.