What is

8.9 + 1.2(3a-1)=14.9 ?

2 answers

multiply each term by 10 to make "nicer" numbers

89 + 12(3a-1) = 149

can you take it from there?
8.9+3.6a-1.2=14.9
3.6a=14.9+1.2-8.9
3.6a=16.1-8.9
3.6a=7.2
divide both sides by 3.6
a=2