Answer to: What is a HDMI port?The HDMI port is used to transfer HD video and multi-channel audio over a single cable.
Answer to: Can I upgrade my RAM emoryYes, you can. According to the Acer website the 5742 will support up to 8Gb of ram.
Answer to: My laptopt can't bootTry removing the battery and the power cord. Hold the power button on for a few seconds. Insert battery and power cord then power on the system and see if that helps.