Make a the subject, 2(3a - c) = 5c + 1

2(3a - c) = 5c + 1 ---> Multiply the brackets out on the left side.
6a - 2c = 5c + 1 ---> Add 2c to both sides
6a = 7c + 1 ---> Divide both sides by 6:

         7c + 1
a =    
             6

2(3a - c) = 5c + 1

Multiply out.

2*3a - 2*c = 5c + 1

6a - 2c = 5c + 1

6a = 5c + 1 + 2c          -2c crosses over to become +2c 

6a = 5c + 2c + 1

6a = 7c + 1      Divide both sides by 6

6a/6 = (7c + 1) / 6

a = (7c + 1)/6

I hope this explains it.

Subtracting is the opposite of adding