Hello ,
substitute 1/3a+b = x and 1/3a - b = y
then the eqs becomes , x + y = 3/4 and x- y = -1/4
Adding the 2 eqs we get x + y + x - y = 3/4 - 1/4
2x = 1/2 => x = 1/4
y = 1/2
ie.., 3a+b = 4 and 3a - b = 2
adding the 2 equations
6a +b - b = 1/4 + 1/2
6a = 3/4
a = 1 b = 1