2t + 3t = 6t is false. The correct answer is 5t.
y + y + y = y(1 + 1 + 1) is true. Correct answer is 3y.
3(2 + x) = 6 + 3x is true. This is an application of the distributive rule for mult.
2a + 2b = 4ab is false. a and b are separate variables with no given relationship to each other. Correct answer would be 2(a+b).
x + 3 = 4x is false. x + 3x = 4x would be true.