To evaluate the expression \( 3a + 4b + a + 2b \) when \( a = 1 \) and \( b = 2 \), substitute these values into the expression:
\[ 3(1) + 4(2) + 1 + 2(2) \]
Now, calculate each term:
\[ = 3 + 8 + 1 + 4 \]
Now, add these values together:
\[ 3 + 8 = 11 \] \[ 11 + 1 = 12 \] \[ 12 + 4 = 16 \]
So, the value of the expression is:
\[ \boxed{16} \]