Equation of one plane is 3x + 4y - 5z = 60 Equation of another plane is 4x + 2y + cz = 0
My thought was to find the normal vector of the first plane. Since I know it is perpendicular to the normal vector of the 2nd plane, I thought I could use dot products, but I was unsure how to find the normal vector of the 2nd plane. Thoughts?
Your idea works just fine. Just think about how we can get the normal vector of a plane from its equation, and take the dot product of both of them, set it to $0$, and then solve for $c$ (you'll end up with a linear equation).