We're talking about points on the unit circle, which are coordinate pairs (x, y) that satisfy
x² + y² = 1
and the coordinates of point on the circle are governed by
x = cos(t)
y = sin(t)
If t = -35π/6, we have
x = cos(-35π/6) = cos(π/6) = √3/2
and
y = sin(-35π/6) = sin(π/6) = 1/2
because both cosine and sine have a period of 2π, which means
cos(θ) = cos(θ ± 2π) = cos(θ ± 4π) = …
and so on; in particular, we have cos(-35π/6) = cos(π/6) because
-35π/6 = π/6 - 6π
So t = -35π/6 corresponds to the point (√3/2, 1/2).