The equation of the parallel line is
y = -3x + 15
y = -3x + 2 is the equation of the original line. Because the lines are parallel the gradient will be the same so we just sub in the give coordinate (4,3) into y = -3x + b
3 = -3 x 4 + b
Then solve for b to get b = 15 and sub that into the equation of the parallel line to get
y = -3x + 15