First you want to claculate the moments
Ix = (1/12)bh^3 - (1/12)bh^3
This is the I of the whole minus the I of the holes
This should yield a value of 93.67in^4
Iy = 2(1/12)hb^3 + (1/12)hb^3
This is the I of the ends plus the I of the middle
This should give you a value of 42.729 in^4
Then calculate your Pcr
Pcr = π
2EI/(KL)
2
E can be found in the tables in the back of the book
I you calculated as 42.729
L is 15ft
K is found in the chapter
This should give you a value Pcr = 377kip
Also if you calculate the area you can find the σcr with σcr=Pcr/A
and if you are given σyield you can see if Eulers formula is valid
If you have any questions let me know.