In (triangle)ABC which equation can you use to find the value of x?

Please Help !!

Option A, cos(57) = x/51

There's a really good mnemonic for remembering how trigonometry works: SOH CAH TOA

Sin = opposite/hypotenuse (SOH)
Cos = adjacent/hypotenuse (CAH)
Tan = opposite/adjacent (TOA)

In this scenario, we have to use cos(57) since 'x' is the adjacent side to the angle.

Since cos = adjacent/hypotenuse, we know that cos(57) = x/51.

Let me know if you need any further explanation.
-T.B.

The answer is 
C.8,664,000,000