An equivalent expression to m+m+m+m that is a product of a coefficient and a variable

Show me how you find x and determine what x is you i appreciate it : )

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

Kayla wants to find the width, ab, of a river. she walks along the edge of the river 65 ft and marks point c. then she walks 25 ft further and marks point d. she turns 90° and walks until her location, point a, and point c are collinear. she marks point e at this location, as shown. (a) can kayla conclude that δ and δ are similar? why or why not? (b) suppose de = 15 ft. what can kayla conclude about the width of the river?