The triangle is "right-angled", which means the length of the hypotenuse is the longest.
Again, it is an isosceles triangle, therefore it'll have two sides equal.
Therefore, except the hypotenuse, the two other sides are equal.
We use pythagoras theorem here because it's a right-angled triangle.
a² + b² = c²
c is the hypotenuse and,
a and b are the two other sides.
We can use a common letter for both sides, say, a.
The equation becomes,
a² + a² = c²
2a² = 4²
2a² = 16
a² = 8 (dividing through by 2)
a = √8
a = 2.828427125
a ≈ 2.8 to 2 significant figures