I didn't understand how we solved for H when we combined the equations :(

Salma,

Let's set up the problem this way. That's 90% of the solving, and I hope it helps you understand more clearly.

Cost of hard drive = HD
Computer with hard drive = CHD
Computer without hard drive = CXD

CHD = CXD + HD; computer can be bought with or without hard drive
CXD = 1950 + HD; computer without the hard drive costs 1,950 dollars more than the hard drive alone
CHD = 2900; The computer with the hard drive costs 2,900

2900 = CXD + HD
1950 = CXD - HD
Two equations, two unknowns. Solve for HD by subtracting 2nd from the first equation:
950 = 2HD
HD = 475

Thanks