Juan is saving to purchase a new computer and has saved $213. he earns $3 a day and needs at least $654 for the computer he would like to purchase. how many weeks will it take him to save for the computer?

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

Which one ? is the answer and how to find this answer

Aspacecraft can attain a stable orbit 300 kilometers above earth if it reaches a velocity of 7.7 kilometers per second. the formula for a rocket's maximum velocity v in kilometers per second is vequalsminus0.0098tplusc ln upper r, where t is the firing time in seconds, c is the velocity of the exhaust in kilometers per second, and r is the ratio of the mass of the rocket filled with fuel to the mass of the rocket without fuel. find the velocity of a spacecraft whose booster rocket has a mass ratio of 20, an exhaust velocity of 2.1 km/s, and a firing time of 15 s. can the spacecraft achieve a stable orbit 300 km above earth?