You must apply this:
SOH CAH TOA
S means sine. O means opposite. H means hypotenuse. C means cos. And T means tan.
Applying this now you would write: cos 55°=80
—-
x
Now solve for x:
cos 55°(x)= 80
————— ——-
cos 55° cos 55°
Now type 80/cos 55° in your calculator and you should get 139.4757 which would round to 139.5