Calculating Specific Mechanical Energy in a spacecraft

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The specific mechanical energy (SME) of an orbit with a semimajor axis of 42,160 km can be calculated using the formula e = -μ/(2a), where μ is the standard gravitational parameter. The user attempted to use the formula -3.986x10^5 / (2 * 42160) but ended up with an incorrect result of -3.36 x 10^10. The correct calculation should yield an SME of approximately -4.73 km^2/s^2, as stated in the textbook. The error likely stems from a misunderstanding or misapplication of the formula. Properly applying the formula will help achieve the correct value for specific mechanical energy.
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Homework Statement



My math is horrible and I'm in an orbital mechanics class which I'm way over my head...my question is...

"What is the specific mechanical energy, e, of an orbit with a semimajor axis of 42,160km"


Homework Equations



I used a formula that took

-3.986x10^5 / 2x 42160

to get the SME.


The Attempt at a Solution



The book says I should get -4.73 km^2/s^2

I get -3.36 x 10^10 ...I'm way off

Can you help me what I'm doing wrong in my formula?
 
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I worked on this and found a formula in the book to solve this. I used

a= -u/2e
 

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