Laws of Sines and Cosines Options

[sheepy]

well i have a project thats like due for this.
and i know how to find the cosines for this.. triangle. a^2 = b^2+c^2 - 2bcCOSA ( the 2's are exponents)

but i don't know how to find the sines.

a^2 = 15^2 + 55^2 – 2(15) (55) (-0.906307787)
a^2 = 225 + 3025 – (1650) (-0.91)
a^2 = 4751.5
a= 68.93

i know the formula for sin is a/sinA = b/sinB = c/sinC ( / is the division, fraction.. whatever you call it line)

so.. does anyone know how to explain or.. teach me really how to find the sin?

thank you so much.