Can someone break this down with answers

Which will form a composite function?

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?

At store a, erasers are sold individually. the cost y of x erasers is represented by the equation y=0.75x. the costs of erasers at store b are shown in the graph.