19 Adar (vayakhel-pikudei)

Meseches Shabbos 97b learns from the words "Aleh" in Shemos 35,1 that says "Aleh Hadevarim asher tziva Hashem La'asos Osam" (these are the things that Hashem commanded that you must do them) that there are 39 melachos because aleh is gematriah 36 the Hei adds 1 and Devarim (things) is plural so adds two. That equals 39. How can we learn that there are 39 things we CAN'T do from a passuk telling us these are the things you MUST do?