Question

How do you do find a? And how can you tell if it is sin, cos, or tan?

Answer 1

30 60 90 triangle
ratio of short leg : hypo = x : 2x

given short leg x = 4 ft
so hypo a = 2x = 2 * 4 = 8

answer
a = 8 ft
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P.S.
if you need to do by using cos sin tan then
Cos (60) = Adj/Hypo = 4/a

a = 4/cos(60) as you know cos(60) = 1/2
so 
a  = 4 /(1/2 = 4 * 2/1 = 8

answer: a = 8 ft

Answer 2

Using SOH CAH TOA so you're finding tye Hypotenuse and you're using the Adjacent side meaning you would use cos :)