3(0.5x+0.4y)+15(0.3x-0.8y)
I'm guessing you want to simplify this.
3 (0.5x + 0.4y) + 15 (0.3x - 0.8y)
= 1.5x + 1.2y + 4.5x - 12y
= 6x -10.8y