The legs of a right triangle are 3 units and 4 units long. another right triangle is dilated from this triangle using a scale factor of 3. what are the side lengths and the perimeter of the dilated triangle?

I believe the last leg will be 5, because it should be a 3,4,5 triangle

you are making it bigger, and so you must multiply 3 to each side (dilate means to become wider)
3 x 3 = 9
4 x 3 = 12
5 x 3 = 15

9, 12, 15 are you triangle side lengths

To find perimeter, add them together

15 + 12 = 27
27 + 9 = 36

Your perimeter is 36

hope this helps