the perimeter is the sum of the 3 sides of the triangle, hence
4x + 10x + 12x = 104
26x = 104 ( divide both sides by 26 )
x = 4
thus the sides of the triangle are
4x = 4 × 4 = 16 inches
10x = 10 × 4 = 40 inches
12x = 12 × 4 = 48 inches
As a check
16 + 40 + 48 = 104 inches ← correct