Some grammatically correct alternatives:
a. “After doing some complicated calculations and working out, I finally found an answer of 137.68638”
The answer I found was 137.68638. There may be other possible answers.
b1. “After doing some complicated calculations and working out, I finally found the answer that is 137.68638”
This probably makes the least sense because it means "Of all the answers, I found that one that is 137.68638".
b2. “After doing some complicated calculations and working out, I finally found the answer. That is 137.68638”
much the same meaning as (e)
c. “After doing some complicated calculations and working out, I finally found the answer, which is 137.68638”
I found the answer and the answer is 137.6838.
d. “After doing some complicated calculations and working out, I finally found the answer to be 137.68638”
The answer I found was 137.68638.
If you are later going to say that you were were wrong or that the answer may not be 137.6838, you should use (a) or (d).
e. “After doing some complicated calculations and working out, I finally found the answer -- 137.68638”
I found the answer. In other words, I found 137.6838.
Before I answer the question can I note that, unless you are transcribing speech, the correct spelling is "going to". In spoken English we often say /ˈɡən.ə/, but in written English always write "going to", unless you are transcribing spoken English.
As noted by an editor of Wiktionary
This spelling, like any nonstandard spelling, risks appearing condescending. Even when going to has the pronunciation that gonna denotes, it is usually spelled . —source
Standard English Grammar requires a form of the verb "to be", so
You gonna lose that key.
is incorrect grammar. It may be acceptable in some dialects, but not in standard English.
As noted above "gonna" is not the standard spelling, so you should write
You're going to lose that key.
Best Answer
Assuming that the failed attempts consisted of red smears over your face, you now managed to apply lipstick in a moving car.
A running car is either a car that is not broken or one whose engine is switched on right now, but does not mean that it is in motion when you pull out your lipstick.