The ratio of the measures of the sides of a triangle is 3:4:6 and its perimeter is 91 inches.
What is the length of the longest side?

42 inch

The perimeter of the ratio triangle is 6+4+3=13
Divide that into the actual triangle perimeter to get the scale 91/13=7

Multiply that scale times the longest side of the ratio (6)
7x6 = 42

To check do the other two as well
7x3=21 and 7x4=28
Add them up to check the perimeter
42+21+28=91

And 42 was the longest side