Find the value of 'k' for which the system of equations 4x + 6y = 7 and 6x + (k + 4)y - 21 = 0 has a unique solution.
Question
Find the value of 'k' for which the system of equations 4x + 6y = 7 and 6x + (k + 4)y - 21 = 0 has a unique solution.