I think you mean "x=1.5" not "x+1.5".
(a) and (b) are two different interpretations of the question.
(a) The given expression (6x+6y)x3 can be written:
6(x+y)x3=6(1.5+2)1.53=6×3.5×3.375=70.875.
(b) The given expression (6x+6y)×3 can be written:
6(x+y)x3=6(1.5+2)×3=6×3.5×3=63.