Question 572185
A = ab*sin(theta)/2


13 = 7b*sin(60)/2


13 = 7b*(sqrt(3)/2)/2


13 = 7b*(sqrt(3)/4)


13*4 = 7b*sqrt(3)


52 = 7b*sqrt(3)


52/(7*sqrt(3)) = b


b = (52*sqrt(3))/21



So the answer is *[Tex \LARGE b = \frac{52\sqrt{3}}{21}]