Determine which theorm should be used, and find CA

Solve: 3x+y=6 4x-2y=18 show all work !

If x varies directly as y, and x = 7.5 when y = 10, find x when y = 4

You have 37 coins that are nickels, dimes, and pennies. the total value of the coins is $1.55. there are twice as many pennies as dimes. find the number of each type of coin in the bank.