I have a laptop power cord that has shorted out, the input is 100-240V--1.5A 50-60 Hz, and the output is 19V=3.42A. I have found a power cord for something else that has an input of 100-240V-50-60Hz 2A, and output of 9V=4A. I am wondering if i could use the one I found for my laptop.
Of course not. If your laptop requires 19 volts at 3.42 amps, but the new adapter only supplies 9 volts at 4 amps, then it will not be sufficient.
Sometimes electronic devices can make do with a bit of variation (a little bit more, a little bit less), but 10 volts is far too much to overcome.
On the bright side, if you have already tried it, you likely did no damage because the new adapter is less than the original one, so it simply did not have enough power. If it was higher, then you could/would have likely burned something.
If you can find another adapter that has an output closer to the original adapter, then it should work, but make sure that the voltage and current match as close as possible, especially if it is higher.
Also make sure that the polarity matches. There should be a symbol to indicate the polarity and it must match or else you could fry your system, even if the adapter is provides less voltage than the device requires.