In any triangle, the smallest angle is opposite to the shortest side.


In the given triangle, the smallest angle is opposite to the 9 cm side and is
concluded between the 11 cm and 13 cm sides.


Write the cosine law for the shortest side of 9 cm


    9^2 = 11^2 + 13^2 - 2*11*13*cos(a)


where "a" is the angle under the question.


From this equation,

    cos(a) =  =  =  = 0.730769231.


So, the angle "a" is


    a = arccos(0.730769231) = 43.04907978 degrees.    ANSWER


You may round it as you want.