You can put this solution on YOUR website!
I have written several Lessons on this site on Linera algebra. They explain this type of question for you.
If your line is perpendicular to the quoted equation, then their 2 gradients (m and n) will satisfy mn=-1.
So, do the following:
1. find the gradient of the quoted line.
2. Find the gradient of your line using mn=-1.
3. put that value into y=mx+c.
4. Find the value of c by also putting into the equation, the point you have been given (-3,8).