The pet store currently has 30 hermit crabs. over the weekend they sold 15 hermit crabs. what was the inventory before the weekend's sales?

Could somebody me with this radical simplification?

Look at the figure below: triangle abc is a right triangle with angle abc equal to 90 degrees. the length of ac is 5 units and the length of ab is 4 units. d is a point above c. triangle adc is a right triangle with angle dac equal to 90 degrees and dc parallel to ab. what is the length, in units, of segment cd?

Using elimination, what is -4x+2y=-12 4x+8y=-24