This is a picture of two lines on an x-y coordinate plane. As you can see, they intersect forming two pairs of vertical angles.

Without referring to the values on the left, try to find a way to determine the acute angle formed between the two lines. These lines are given as: -x + y = 0 and -2x + 3y = -9. This is an exercise that involves both algebra and trigonometry. When you are done, check your answer with the one on the left. Have FUN!