Dividing each side of the equation 3r=18 by 3 gives r=6. Substituting 6 for r in the expression 6r+3 gives 6(6)+3=39. Alternatively, the expression 6r+3 can be rewritten as 2(3r)+3. Substituting 18 for 3r in the expression 2(3r)+3 yields 2(18)+3=36+3=39. Choice A is incorrect because 6 is the value of r; however, the question asks for the value of the expression 6r+3. Choices B and C are incorrect because if 6r+3 were equal to either of these values, then it would not be possible for 3r to be equal to 18, as stated in the question.