The moons gravity can be used too slingshot a capsule back,why Carnot we send one into orbit the same way?

No we cannot use the Earth as a slingshot to send a space craft into orbit. You first have to attain escape velocity from Earth of 11.2 km/second. The slingshot mechanism is used when a space craft has already attained its maximum velocity and once in orbit another planetary m's gravity is used to send the space capsule around this planetary m and the result is an increase in the velocity of the space craft.