I hope this is in the right place.

Prove the following statement by a direct proof of the contrapositive statement.

If two lines are perpendicular, the product of their gradients is -1.

The contrapositive would be:

If the products of two gradients is not -1, then their lines are not perpendicular.

My problem is I don't know how to go about proving the statement.

I tried this:

is not equal to 2

But I don't know how to continue.

Thanks