If the determinant of that matrix is not 0, you can't- that matrix equation has a single solution which may or may not satisfy that additional condition. If the determinant is 0 (I haven't checked but I imagine that's why you are asking) then probably the best thing to do is to go ahead and "try" to solve the equation- since the determinant is 0, you won't be able to find a single solution but will be able to reduce to either a single equation in l, m, and n (if the matrix has rank two) or two equations relating them (if it has rank 1). In the first case, you can include your constraint with the single equation to get a one parameter solution, in the second, you can add it to the two equations to get three conditions that you can solve for a specific point (since that constraint is quadratic, there may be no solution).