M square - 9 / 4 M-4 × (1 + m square - 8m + 16 / 10m-19) △ M-3 / 1

M square - 9 / 4 M-4 × (1 + m square - 8m + 16 / 10m-19) △ M-3 / 1

=(m-4)/(m+3)(m-3) ×(m²+2m-3)/(m²-8m+16)×(m-3)
=(m-4)/(m+3) ×(m+3)(m-1)/(m-4)²
=(m-1)/(m-4)