How to do linear equations 27, 28,28,30

Are you solving for A? Simply cross multiply and solve for A

2a - 3       a - 1
 =  
    3             2

you want to mutiply (2a -3)(2) and (a - 1)(3)

(2a - 3)(2) = (a - 1)(3)
4a - 6 = 3a - 3

Now get the a terms on the same side

a - 6 = -3

Now move the 6 over
a = 3

28 and 29 are done the same way. For #30 you need to move the 1/4 from the left side to the right side before solving (so it looks like below) then cross multiply as you did in the other problems.

3/x = 3/4