a)
[tex]5-(2-7x)=5-2+7x=\boxed{7x+3}[/tex]
b)
[tex]8+3a+2(5-a)=8+3a+10-2a=\\\\=3a-2a+8+10=\boxed{a+18}[/tex]
c)
[tex]-2(c+6)+4(7-b-3c)=-2c-12+28-4b-12c=\\\\=-4b-2c-12c-12+28=\boxed{-6b-12c+16}[/tex]
