Write an expression with 4 terms, containing the coefficients 3, 6, and 9

Let the variable used in the expression be "x".

Since the desired expression should contain 5 terms, therefore the powers of "x" will be as follows:
x^0 , x^1 , x^2 and x^3 where x^0 = 1 and x^1 = x

Now we are given only 3 coefficients to use, so we will have to assume a fourth one.
Assume the fourth coefficient to be 7

It is a convention to write the powers in an expression in descending order, so the expression will be as follows (without coefficients):
x^3 + x^2 + x + constant

Now add the coefficients in any desired order since there are no conditions, the final expression can be written as follows:
3 x^3 + 6 x^2 + 9 x + 7