OK.
So if you have an isosceles triangle, two of the three sides are congruent. So this means that the two legs are congruent to each other.
This also means that the base angles of the triangles are congruent. And since the angles in a triangle have to be supplementary and you KNOW that one of the angles is 90...
180-90=90. Divide 90 by 2 (so you can get the measure of the two base angles), and your quotient is 45. Each base angle measures 45 degrees.
Because of this, we have a special right triangle...45-45-90.
The hypotenuse is radical 2 times longer than either leg.
OK, now that we know that...
0.7071 is the square root of two divided by two. Keep that in mind as I give you the equation!
x(radical two)=0.7071(x+3)
Divide each side by the square root of two now.
x=0.7071x+2.1213/1.414
x=(1/2 x) + 2.1213
This means that x=4.2426.
SO EACH LEG EQUALS 4.2426 or 4.24. TA-DA!
Hopefully you understand how I got there!