The perimeter is the sum of the length's of the sides of the triangle.
So 2X + 3X -1 + X + 12 = 41
Now combine like terms--> 6X + 11 = 41
Subtract 11 from both sides--> 6X = 30
Divide both sides by 6--> X = 5
Now you can plug 5 back into the original problem to calculate the lengths of the 3 sides. Then add them up to make sure you get 41.