Vectors on straight line finding value of p The position vectors of points A and B relative to an origin O are given by
$$\overrightarrow{OA}=\pmatrix{p\\1\\1}\qquad\text{and}\qquad\overrightarrow{OB}=\pmatrix{4\\2\\p},$$
where $p$ is a constant. 
(i) In the case where OAB is a straight line, state the value of p and find the unit vector in the
direction of OA.[3] 
How to find the value of p(using a mathematical sense), i try to remove constant OB out giving(2) out....

Vector OB is some  constant multiple of vector OA.
$$\vec{OA} = \lambda  \vec{OB}$$
After this it's very simple, you have three equations and two unknowns to solve.
