Function Form is written as: $y=$ 
How many of the following equations are written in function form?  ($1$) $2x-y=3x+1$;~($2$) $2x=-5y+2$;~($3$) $y=7x+12$;~($4$) $y+6=12x-17$;~($5$) $y=x+z+ab$ 
  $$1$$ 
  $$2$$ 
  $$3$$ 
  $$4$$ 
  $$5$$ 