Answer: 48 degrees
=============================================
Work Shown:
Apply the law of cosines
a^2 = b^2 + c^2 - 2*b*c*cos(A)
10^2 = 13^2 + 11^2 - 2*13*11*cos(A)
100 = 290 - 286*cos(A)
100-290 = -286*cos(A)
-190 = -286*cos(A)
190 = 286*cos(A)
286*cos(A) = 190
cos(A) = 190/286
A = arccos(190/286)
A = 48.368620460647
A = 48
Make sure your calculator is in degree mode.