Add the rational expressions. \[ \frac{18 a-1}{3 a+3}+\frac{13-6 a}{3 a+3} \]

Step 1 ：Combine the numerators: \(18a-1+13-6a\)

Step 2 ：Simplify the numerator: \(12a+12\)

Step 3 ：Factor out the common factor in the numerator and denominator: \(\frac{12(a+1)}{3(a+1)}\)

Step 4 ：Cancel out the common factor: \(\frac{12}{3} = 4\)

Step 5 ：The simplified sum of the rational expressions is \(\boxed{4}\)