Me ! i’m so dumb and don’t understand this !

If you are trying to move an attached term across the equal sign (=), what operation would you use to move it?

Expresa commuter train leaves downtown sation and travels at a average speed of 55 miles per hour towards the north side sation, which is 50miles away. thirty minutes later, express commuter train #7 leaves north side and travels at a average speed of 35miles per hour towards downtown sation. at the moment the two trains pass each other, how far(in miles) is train #12 from the downtown sation and how long(in minutes) has the #12 train been traveling

Which expression is equivalent to -5(3x - 6/7)