Answer:
[tex]=21[/tex]
Correct option is A
Step-by-step explanation:
[tex]-3\left(5-12\right)\\Follow\:the\:PEMDAS\:order\:of\:operations\\\mathrm{Calculate\:within\:parentheses}\:\left(5-12\right)\::\quad -7\\5-12=-7\\=-3\left(-7\right)\\\mathrm{Multiply\:and\:divide\:\left(left\:to\:right\right)}\:-3\left(-7\right)\::\quad 21\\-3\left(-7\right)\\\mathrm{Apply\:rule\:}-a\cdot \left(-b\right)\:=\:a\cdot \:b\\-3\left(-7\right)=3\cdot \:7=21\\=21[/tex]