How do you rotate a triangle counter clockwise 90 degrees?

Counterclockwise is to the left (the opposite of how the clock turns). To rotate the triangle in a full circle it would be 360 degrees, so 90 degrees would be a quarter of a circle.

Picture a round clock with numbers (not a digital clock). Place the triangle within that circle with any corner at 12 O'Clock. Rotate the triangle so that the selected corner is now at 9 O'Clock. Clockwise refers to these clocks with hands that always move (at the top) from left to right). Counter Clockwise would be the opposite direction. A circle consists of 360 degrees. 12 O'Clock = 360 or 0 degrees.
90 degrees = 1/4 of the circle which could be either 3 O'Clock or 9 O'Clock. The direction (clockwise or counter clockwise) tells you which way to go.