we have that
points (−20, 21) and (0, 0)
Let
A----> (−20, 21)
B-----> (0, 0)
using a graph tool
see the attached figure
step 1
find the distance AB
applying the Pythagorean theorem
AB²=20²+21²--------> 841---------> AB=√841-------> 29
AB=29 units
cos Θ=20/29-------> 0.6897
the angle Θ belong to the second quadrant
therefore
the cosΘ is negative
the answer is
the cos Θ is -0.6897
