Sub QRBKSB(A(), N, Q(), B(), X())
    For I = 1 To N
        Sum = 0#
        For J = 1 To N
            Sum = Sum + Q(I, J) * B(J)
        Next J
        X(I) = Sum
    Next I
    For I = N To 1 Step -1
        Sum = X(I)
        For J = I + 1 To N
            Sum = Sum - A(I, J) * X(J)
        Next J
        If A(I, I) = 0# Then Print "A is singular matrix.'"
        X(I) = Sum / A(I, I)
    Next I
End Sub