How do I solve for this law of sine to find theta? The part 90+theta confuse me.

Question

(sin(90+theta)/700)=(sin(105)/960)

Answer 1

sin(90+x) = -cosx
sin(105) = -cos(75)
so we have
cosx/700 = cos75/960
960cosx = 700cos75
cosx = 700/960cos75
. . .