check the picture below on the left-side.
so, negative angles move "clockwise", namely how the hands on a clock move, so -7π, is really -2π-2π-2π-π, which means, we go around the circle clockwise 3 times, as you see there in the green lines, and then we go a little bit more, namely -π, and that lands us on the -7π, or -π or the positive counterpart of π angle, same terminal point for all three.
and since tan(π) = 0, then tan(-7π) = 0, and likewise tan(-π) =0.
check the picture below on the right-side.
as you see there, the -3π/4 angle, will land there, namely is the same terminal point as the 5π/4 positive counterpart.
