Beta error

Hey guys

I really need help from you to calculate the beta error of two different samples.

The values are:
Average rate of the first sample: 61.083
Average rate of the second sample: 48.083
Difference (second minus first): -13
Standard deviation (calculation with the assumption that the samples are interdependent): 9.146

Thus, I get a z value of -1.42 ((-13-0)/9.146) --> 7.8% = alpha error

I want to calculate the beta error with an alpha error of 10%. Can somebody please explain me the steps to get the result? I would really appreciate that!!!

Best regards,