How to allign two vertexes to a third one with s y 0 command

First put the transform pivot point to the active element on the top bar.
Then select the two vertexes that needs to be alligned to a third one (or fourth, sixth..etc).
Now select the vertex (it will have a different color, compared to the other selections as it will be the active object)
Finally, use the command sy0 and the first two vertexes will be alligned to the third one on the y axis.