Calculating Specific Mechanical Energy in a spacecraft

[spacemike1]

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?

 

[spacemike1]

I worked on this and found a formula in the book to solve this. I used

a= -u/2e