The equation x² - 11x + c = 0 has two real solutions that differ by 7. What is the value of c?

Let the roots be $r$ and $r + 7$. The sum of the roots of $x^2 - 11x + c = 0$ is 11, so $2r + 7 = 11$ and $r = 2$. The roots are 2 and 9, and the product is $c = 2 \times 9 = 18$, choice A. Check: $x^2 - 11x + 18 = (x - 2)(x - 9)$. Choice B (28) comes from using 7, the difference, as one of the roots, giving roots 4 and 7 that differ by 3. Choice C (11) is the sum of the roots. Choice D (4) is $\frac{11 - 7}{1}$, a difference misread as the product. The constant term equals the product of the roots.