First evaluate the left-hand side expression for y = 3:
3 + 2 = 5
Then evaluate the right-hand side expression for y = 3:
2(3) = 6
Since 5 is not equal to 6, the left-and right-hand sides of this equation do not evaluate to the same number when y = 3. Therefore, y = 3 is not a solution to the equation y + 2 = 2y. Adios, y = 3. Go peddle your falsehood to some other sucker.
Be Careful: Work with the left-hand side and the right-hand side of the equation separately. Otherwise, you are in danger of writing false equations or making a wrong turn. You don't want to make a wrong turn, because then it will take your GPS like a minute and a half to recalculate.