The points are X(2,−1), Y(3,k), Z(1,4).
For collinear points, they all lie on the same straight line. This means the slope between any two pairs of points must be equal.
For X(2,−1) and Y(3,k):
Slope of XY=3−2k−(−1)
=1k+1
=k+1
For X(2,−1) and Z(1,4):
Slope of XZ=1−24−(−1)
=−14+1
=−15
=−5
Since the points are collinear:
k+1=−5
k=−5−1
k=−6
Therefore, k=−6.