If f(x)=3x+2, then f(2a+b)=?

To determine the value of the fuction f, at the point x =
2a + b, we'll have to replace x by the expression
2a+b:

f(2a+b) = 3*(2a+b) +
2

We'll remove the
brackets:

f(2a+b) = 6a + 3b +
2

The requested value of the function at the
point x = 2a + b is f(2a+b) = 6a + 3b + 2.