The line segment with endpoints (−20, 21) and (0, 0) is graphed on the standard coordinate plane and makes an angle, θ, with respect to the x axis. what is cos θ?

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