First collect like terms, the terms with a's as;
9-7a +5a = -11
9- 2a = -11
Take 2a to the right-hand side and -11 to the left-hand side as
9 + 11 = 2a ----------take note of the change in signs, - to +
20 = 2a ---------------divide both sides by 2 to get value of a as
20/2 = 2a/2
10 = a