If and then find the projection of on

Find λ if the vectors and are coplanar.

If a line makes angles 90^o, 60^o and θ with x, y and z-axis respectively, where θ is acute, then find θ.

Write the element a₂₃ of a 3 × 3 matrix A = (a_jj) whose elements a_ij are given by

Find the differential equation representing the family of curves where A and B are arbitrary constants.