What number best completes both equations

The depth of a lake at a certain point, wich is a function of the distance of that point from shore, is 30 feet.

Lorne subtracted 6x3 – 2x + 3 from –3x3 + 5x2 + 4x – 7. use the drop-down menus to identify the steps lorne used to find the difference.

Multiply. write your answer in simplest form. 3/8 x 5/7