Correct answer: 536 Hz
The beat frequency is the absolute difference between the frequencies of the two strings. Initially, the beat frequency is 6 Hz, so the frequency of B could be either 530 + 6 = 536 Hz or 530 - 6 = 524 Hz. When the tension in B is decreased, its frequency decreases, and the beat frequency increases to 7 Hz. This implies that the original frequency of B must have been 536 Hz, as decreasing it would increase the beat frequency with A.