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.
