Explanation:
this creates a right-angled triangle.
the right angle (90°) being the angle between flag pole and ground.
the angle ground-wire is 50°.
because we know that the sum of all angles in a triangle is always 180°, we know then that the angle flagpole-wire is
180 - 90 - 50 = 40°
so, we have the situation to know 1 side and all angles.
to get the other sides we use best the law of sine :
a/sin(A) = b/sin(B) = c/sin(C)
where the sides and related angles are always opposite of each other.
in our case now we have
24/sin(50) = wire/sin(90) = wire/1 = wire = 31.32977494... ft
so, the rounded answer is that the wire must be
31.3 ft long