To solve the equation for k in the expression 4a + 9ab, we first need to clarify if the equation is presented in the format of an equation (like k = …) or just an expression that needs to be manipulated. However, since you mentioned solving for k, we will consider 4a + 9ab = k as a starting point.

Here are the steps to isolate k:

1. Start with the given expression:

   4a + 9ab = k

2. Identify the terms: In this expression, you can see that 4a and 9ab are combined to give you the value of k.
3. Combine like terms if applicable: In this case, 4a and 9ab do not have like terms since one involves only a and the other involves b as well. Thus, you leave them as they are.
4. Final expression for k: You can express the result as:

   k = 4a + 9ab

Now, you can use this expression to find the value of k for any specific values of a and b you might have.

If you have additional conditions or values for a or b, please provide them, and we can explore further!