3x(a+b)+y(a+b)

To simplify the expression \( 3x(a + b) + y(a + b) \), you can factor out the common term \( (a + b) \):

\[ 3x(a + b) + y(a + b) = (3x + y)(a + b) \]

So, the simplified expression is:

\[ (3x + y)(a + b) \]