check the picture below.
notice, that one go-around is 2π, or namely 4π/2, and if we go 3π/2 more over, we'd end up at at 4π/2 + 3π/2 or 7π/2.
a couple of coterminal angles will be those in blue, one angle in the first go-around, and another one by simply going around one more time by 4π/2 extra, 7π/2 + 4π/2 is just that one there.
