Please Help, I really need this fast and i do not know how to do this!

Two similar right triangles have a scale factor of 1 : 3.
If the smaller right triangle has a perimeter of 21 m, find the perimeter of the larger right triangle.

Question

Please Help, I really need this fast and i do not know how to do this!

Two similar right triangles have a scale factor of 1 : 3. 
If the smaller right triangle has a perimeter of 21 m, find the perimeter of the larger right triangle.

directrix · Answer

If two triangles are similar, the scale factor (ratio) is the same as the ratio of the perimeters of the two triangles.

So, 1 is to 3 as 21 is to x where x is the perimeter of the larger triangle.

1/3 = 21/x

Solve for x and that's the perimeter.