Making a sketch always help.
Draw a horizontal arrow, say towards the right and label it 300 N
Draw a vertical line from the tip of the arrow, downwards and label it 575 N
Now, complete the triangle by drawing an arrow starting from the left tip of the horizontal arrow, towards the arrowhead of the vertical arrow.
You can compare the forces as lengths too, meaning that you apply Pythagoras' Theorem to get the resultant.
Then, the angle with the horizontal can be easily found. The most appropriate way is to use the tan ratio. Can you give it a try?