Negative & Positive Shear Factor

sorter

My question relates to constructional geometry & matrices aren't to be involved in the solution because stated Math level is up to O Levels... The figure below shows shear with y=3 as invariant line & shear-factor of 3




My question is if you are provided the original polygon & asked to do shear with y=3 as invariant & shear-factor 3 or -3 how would I know whether to slide segment AD to right or left? Same confusion thus occurs with segment BC?

Moreover shear-factor is defined via (object-image dist)/(object-invariant dist) thus if the object polygon ABCD & its image say the red one is given & you are asked to completely define the transformation I can give the invariant line & the factor's magnitude but I can't tell the sign (+ or -) of the shear-factor because distances are always positive? Plz help this teacher