Find the answer - 1=1/2a+9

-1=1/2a+9

move + 9 to the other side

sign changes from + 9 to - 9

-1-9=1/2a+9-9

-1-9 = 1/2a

-10 = 1/2a

Mutiply both sides by 2/1

(-10) (2/1) = 1/2a*2/1

Cross out 2 and 2, divide by 2 and becomes 1

Cross out 3 and 3, divide by 3 and becomes 1

1*1*a = a

a = - 20

Answer: a = - 20