[tex]\displaystyle\bf\\\Big(ab-2b\Big)\Big(2a+3b\Big)-\Big(2a-5ab\Big)\Big(b+4a\Big)=\\\\=ab\cdot 2a+ab\cdot 3b-2b\cdot2a-2b\cdot 3b-\Big(2a\cdot b+2a\cdot4a-5ab\cdot b-5ab\cdot 4a\Big)=\\\\=2a^{2} b+3ab^{2} -4ab-6b^{2} -2ab-8a^{2} +5ab^{2} +20a^{2} b=\\\\=22a^{2} b+8ab^{2}-8a^{2} -6b^{2} -6ab[/tex]