Given:
the perimeter of an isosceles triangle 30cm
The length of each congruent side is 3cm more than the length of its base.
Let the length of the base side = x
So, the length of each congruent side = x + 3
the perimeter is the sum of the lengths of the sides of the triangle
So,
Solve the equation to find the value of x
So, the length of the base side = 8 cm
And the length of each congruent side = 8 + 3 = 11 cm
So,
The lengths of all sides are: 11 cm, 11 cm and 8 cm