In a 30-60-90 triangle, if they give me b=15, what would be a & c?

I think you should just know ratios for this. If its 30-60-90 and b= 15, than A= 15/2=7.5 You get this because A is half the size of B. C is three times the size of C, so 7.5 x 3 = 22.5

So 7.5,15,22.5

Hope this helps.