If we're assuming that the values you gave are side lengths, then you just need to set two of them equal to each other and solve for x. You can do this because each side in an equilateral triangle is the same length.
2x - 3 = x + 5
x - 3 = 5
x = 8
Now you can just plug x = 8 into any of the three side lengths (since they're all equal) to find the length of one side:
2(8) - 3
16 - 3
So one side of this triangle has a length of 13. Since each side is the same length and a triangle has 3 sides, you can just multiply 13 by 3 to get the total perimeter of the triangle.
13 x 3 = 39
The triangle's perimeter is 39.