a = 2x - 18
remove the parenthesis and simplify
x - 3 + x - a = 15
2x - 3 - a = 18 ( add 3 to both sides
2x - a = 18 ( subtract 2x from both sides )
- a = 18 - 2x ( multiply through by - 1 )
a = - 18 + 2x
