explain the following solution step by step please:

10 Given that A sina, Wb/m, find H at point (4, 60%, 30"). Solution: B = VxA=- (4, sin #) - dAg 1 04 I sin 8 r sine dg ar (rA)de + (TAB) ae 1 10 1 a 2 sin & cos ea, - (10) sin ed + 0a rsine r 20 B = cos da, At (4, 60', 30'), r = 4, 0=60 B 20 H =- cos 60'a = 4.974x10 a, A/m Ho 47 x107