Can you help me with this?

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

Hector and alex traded video games. alex gave hector one fourth of his video games in exchange for 6 video games. then he sold 3 video games and gave 2 video games to his brother. alex ended up with 16 video games. how many video games did alex have when he started?

9/10 divide by -3/15 as a mixed number