Given points are A(−1,3), B(2,k), and C(5,−1).
For three points to be collinear, the slope between any two pairs of points must be equal.
For points A(−1,3) and B(2,k):
Slope of AB=2−(−1)k−3
=3k−3
For points B(2,k) and C(5,−1):
Slope of BC=5−2−1−k
=3−1−k
For collinearity, slope of AB= slope of BC:
3k−3=3−1−k
k−3=−1−k
k+k=−1+3
2k=2
k=1
Therefore, the value of k=1.