4. Let a=(1,?3,1) and x=(x1?,x2?,x3?). Write down the dot product a?x as a matrix multiplication Ax. The solutions of Ax=0 are the vectors in R3 that are perpendicular to the vector 5. Draw the row and column pictures for the equations x?3y=4 and x+2y=?1. 6. Use backward substitution to solve the following two systems of equations: x1?+2x2?=22x1??x2?+3x3?=?2?x2?=3 and ?2x2?+x3?=0?3x3?=?6?