Question

20 - 10y - 7y + 9

Answer 1

29-3y, because 20+9, and 10y-7y :)

Answer 2

Suppose you want to equate an expression to zero:

$\[\begin{array}{l}20 - 10y - 7y + 9 = 0\\29 - 17y = 0\\17y = 29\\y = \frac{{29}}{{17}}\end{array}\]$