Question 28380
so you have to isolate the "a"
switch the equation around for  mathematical purposes

2a-ab+3=y

subtract both sides by 3

2a-ab+3-3=y-3
this is 
2a-ab=y-3
factor out an "a"
a(2-b)=y-3
divide both sides by "2-b" to isolate the "a"
a=(y-3)/(2-b)