4.2×1.2-5.2×0.7
=(5.2-1)×1.2- 5.2×0.7
=5.2×1.2-1.2-5.2×0.7
=5.2×1.2-5.2×0.7-1.2
=5.2×(1.2-0.7)-1.2
=5.2×0.5-1.2
=2.6-1.2
=1.4