The answer is very simple .
First you need to determine the slope, which is the reciprocal of the first line.
Then,
y = mx + b
m => -1/m
The perperdicular line has a new slope equal to m = -1/2
In the formula (y - y1) = m ( x - x1 ) + b , you should sustitude this value.
So the final equation will be
-3 = -1/2 (0) + b
b = - 3
y = (-1/2)x - 3
That is the solution
