Mars can't be terraformed. Sure, it has lots of CO2 that "could" be converted into breathable oxygen, BUT there is no nitrogen. Our atmosphere is mostly Nitrogen, which is why we breath out almost as much as we breath in. Also, there will be no plant life because of, again, no Nitrogen.
Now, Venus, on the other hand is ripe for terraforming. All we would need to do is convert that CO2 into oxygen, which would, hopefully, reverse the extreme greenhouse effect happening there. But, if we can't do anything about the tiny amount of CO2 in our atmosphere, we have 0 hope of making a dent in Venus's !!!
GRAMMAR: The difference between knowing your shit and knowing you're shit.