If points A(x,−1),B(2,1) and C(4,5) are collinear, Then, Slope of AB= Slope of BC ⇒2−x1−(−1)​=4−25−1​⇒2−x1+1​=24​⇒2−x2​=2⇒2=4−2x⇒2x=4−2⇒x=1​