-1+3-5+7-...+97-99 plz help me

What does it mean by solve the exponential equation for x?

What number should be added to both sides of the equation to complete this square? x^2-10x=7

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