The answer depends on what information you have but, basically, you need to add together the lengths of the three sides of the triangle.
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The perimeter of any triangle is the sum of its 3 sides.
If the base of an isosceles triangle is 11 and its perimeter is 39, then it has a height of 12.87.
Assuming that neither of the sides of equal length is the base, Perimeter = Base + 2*Slanted sides.
It is the sum of the lengths of its sides.
Perimeter of the isosceles triangle: 16.2+16.2+8.1 = 40.5 cm