is not a special angle so we construct a right triangle with one point at the origin, one point at (-1,0) and the hypotenuse of the triangle is 4. This is because cos = x-value/hypotenuse.

We can use pythagorean's theorem to find the other leg of this right triangle.

with x = -1

Your told , and roughly speaking tan = y-value/x-value. Because the x-value is negative we know the y-value needs to be negative to make the fraction y/x positive.

So your triangle is in the quadrant III and now you have a triangle from which to get the other trig functions.