[tex]\boxed{\begin{array}{lr} a)\\3x-40=x-42\\3x-x=-42+40\\2x=-2\\x=\frac{-2}{2}\\x=-1 \ \ \checkmark \end{array}}\\\\\boxed{\begin{array}{lr} b)\\15-3x=x+19\\-3x-x=19-15\\-4x=4\\4x=-4\\x=\frac{-4}{4}\\x=-1\ \ \ \checkmark \end{array}}\\\\\boxed{\begin{array}{lr} c)\\10x-6x+8=x-2x\\4x-x+2x=-8\\5x=-8\\x=\frac{-8}{5} \end{array}}[/tex]
Resposta;
só a c) que o resultado não é -1
A) -1
b) -1
