Find the sum of (6x2 - 9x + 7) and (- 8x2 + 10x - 14)

I’m really confused and need to learn this

Sam opened a money-market account that pays 3% simple interest. he started the account with $7,000 and made no further deposits. when he closed the account, he had earned $420 in interest. how long did he keep his account open?

Jonathan puts $400 in a bank account. each year the account earns 6% simple interest. how much interest will be earned in 5 years?