Please help me find the correct answer. My answer is incorrect

At one point, certain U.S. Treasury bonds were callable. Consider the prices in the following three Treasury issues as of May 15, 2017: 05/15/2020 6.10 05/15/2020 7.15 05/15/2020 8.00 108.46875 108.53125 - .31250 3.100 109.00000 109.06250 -.09375 3.920 114.06250 114.25000 -.46875 3.000 The bond in the middle is callable in February 2018. What is the implied value of the call feature? Assume a par value of $1,000. (Hint: Is there a way to combine the two noncallable issues to create an issue that has the same coupon as the callable bond?) (Do not round intermediate calculations and round your answer to 2 decimal places, e.g., 32.16.) Call value 28.72