3(5-2a)^2-4(3a-5)(7-a)+15a^2-7=
=3(5-2a)(5-2a)- 4(21a-3a^2-35+5a)+15a^2-7=
=3(25-10a-10a+4a^2)-104a-12a^2+140+ 15a^2-7=
=75+140-7+12a^2-12a^2+15a^2-104a=
= 208+15a^2-104a
3(5-2a)^2-4(3a-5)(7-a)+15a^2-7=
=3(5-2a)(5-2a)- 4(21a-3a^2-35+5a)+15a^2-7=
=3(25-10a-10a+4a^2)-104a-12a^2+140+ 15a^2-7=
=75+140-7+12a^2-12a^2+15a^2-104a=
= 208+15a^2-104a