As the general formular for a straight line is
m represents the gradient
c represents the point at which the line crosses the y axis.
Therefore you simply plug the numbers into the equation.
You can work out whether 2 lines are perpendicular by multiplying their gradients. The product of the gradient of perpendicular lines will always be -1.
If lines are perpendicular, M1× M2 = − 1
So you can rearrange the equation to get M2=-1/M1.
As they perpendicular they have to intercept at the same point on the Y axis therefore you get the equation