Answer
The semimajor axis is $~~6640~km$
Work Step by Step
We can find the distance from the center of the Earth to the closest point:
$d_1 = 6370~km+180~km = 6550~km$
We can find the distance from the center of the Earth to the farthest point:
$d_2 = 6370~km+360~km = 6730~km$
We can find the total distance between the two extreme points:
$d = 6550~km+6730~km = 13,280~km$
We can find the semimajor axis:
$\frac{d}{2} = \frac{13,280~km}{2} = 6640~km$
The semimajor axis is $~~6640~km$
