You have the following equation:

if you divide by 6 both sides, you obtain:

the slope of the previous line is m = 1/6.
Take into accout that the relation between the slopes of two perpendicular lines is given by:

For the required perpendicular line you have:

Now, use the following general equation for a line:

where (xo,yo) is any point on the line.
Replace (xo,yo) = (3,5) and m' = -6 into the previous formula and solve for y:

Hence, the equation of the perpendicular line is, in intercept slope form:
y = -6x + 23