Solution: 3x=57- 18

3x=39

x=39/3

x= 13

0.8x-6+42 is not an equation, not an equation.

2x+40= 100

Solution: 2x= 100-40

2x=60

x=60/2

x=30

I finished it for you. I am in the fourth grade. The equation must contain unknowns and must be an equation.