Triangle 1 and 2 are similar to the sides of the first: 6,7,8 its perimeter 21

Triangle 1 and 2 are similar to the sides of the first: 6,7,8 its perimeter 21 sides of the second: x, y.z its perimeter 105 find the sides of the second triangle.

By the condition of the problem, the perimeters of both triangles are known. We find their ratio – this will be the coefficient of similarity.
k = P1 / P2 = 21/105 = 0.2.
Knowing the sides of the first triangle and the similarity coefficient, we find the sides of the second triangle:
x = 6 / 0.2 = 30;
y = 7 / 0.2 = 35;
z = 8 / 0.2 = 40.
Verification:
P2 = 30 + 35 + 40 = 105, 105 = 105.
Answer: the lengths of the sides of the second triangle are 30, 35, 40.