If θ1 and θ2 are the angles made by the two straight lines with x-axis, then the angle between the two straight line is given by

θ= θ2 - θ1.

Τhen tanθ = tan(θ2 - θ1) = (tanθ2 - tanθ1)/( 1 +tanθ1*tanθ2) = (m2 - m1)/(1 + m1*m2)

If m1*m2 = -1, tanθ becomes infiifnty which is possible when θ = π/2.