A scalene triangle is a triangle that has 3 sides that are all different lengths.
Let these letters represent the problem:
a = 8.7
b = side 2
c = side 3
P = 54.6
To find the perimeter, we just need to add all the sides [ P = a + b + c ]
So, put what we have in the formula above.
54.6 = 8.7 + b + c
Best of Luck!