Answer and explanation:
1) 3w+7=19;
3w=19-7;
3w=12;
w=12/3;
w=4.
2) 9/3+4=6 - invalid expression, since 9/3=3, hence 3+4=7 and cannot be equal to 6.
3) -3=12y-5(2y-7);
-3=12y+(-5)*2y+(-5)*(-7);
-3=12y-10y+35;
-3=2y+35;
2y=-3-35;
2y=-38;
y=-38/2;
y=-19.