Remark
The slopes of perpendicular lines = -1 when multiplied together. So you represent this as m1 * m2 = - 1. This formula applies if and only if the lines are perpendicular.
So in this case if m1 = 2/5
then m2 can be found as
m1 * m2 = - 1
2/5 * m2 = - 1 Multiply both sides by 5
2*m2 = -1 * 5 = - 5 Now divide by 2
m2 = -5/2
Answer: A