4. 20 pts] Solve the equation -2 3 2 61=0 z-4 1 2r for r. (Hint: First compute the left determinant and express it as a quadratic function of z in the form of az bz c, where a, b, and c are real numbers. (One method to compute this determinant is to use a cofactor expansion across a row.) Then solve the equation ax2 + bz + c-0.)