Here’s a good article that explains how to use the law of cosines to find angles in a triangle when all 3 sides are given.
The largest angle C will be the angle formed from the 2 sides a=4u and b=5u. The longest side c=8u will form the base of the triangle.
Hence, cos (C) = (a2 + b2 – c2) / 2ab = (42 + 52 – 82) / 2(4)(5) = -0.575
Hence C = cos-1(-0.575) = 125.1o
#Corrected 9 Oct.