A perpendicular line has the opposite slope to the given line, which has a slope=2, the x coefficient. The perpendicular slope is -½, because the product of the two slopes has to be -1.
We can use the slope-intercept form of a line to find its equation, plugging in the given point:
y-4=-½(x-1), y=-½x+½+4=-½x+9/2. This can be written in different ways:
2y=9-x, 2y=-x+9, x+2y=9, x+2y-9=0, y=½(9-x). All these are equivalent.