two lines have equations y=m(1)x + c(1) and y = m(2)x + c(2), and m(1)m(2) = -1, proove that the lines are perpendicular.

im aware that this is supposed to be an extension question but still i dont know what to use in order to proove this, seeing that m(1)m(2) = -1 is apparently not enough. can somebody help with a clear proof on how to do this?