Answer:
The length of the shortest side of the triangle is 10 units.
Explanation:
Let a be the shortest side of the isosceles triangle and b be the two congruent sides.
The congruent sides b are each one unit longer than the shortest side. Hence:

The perimeter of the isosceles triangle is given by:

This is equivalent to the perimeter of a square whose side lengths are two units shorter than the shortest side of the triangle. Let the side length of the square be s. Hence:

The perimeter of the square is:

Since the two perimeters are equivalent:

Substitute for b:

Solve for a. Distribute:

Simplify:

Hence:

The length of the shortest side of the triangle is 10 units.