There are only four triangles each occupying a corner of the square encompassing the octagon. The internal angle of each of the eight isosceles triangles inside the octagon is 360/8=45 degrees. So the other two angles are (180-45)/2=67.5 degrees. Each of the four external triangles is a right-angled isosceles triangle, so the other angles are 45 degrees and the hypotenuse is the length of the side of the octagon. The four hypotenuses are BC, DE, FG and HA. If each octagon triangle has a base length of 10 (side length of the octagon) then the hypotenuse of the corner triangles is 10 and the other two sides are given by x in the equation 2x^2=100 (Pythagoras), so x=sqrt(50)=7.071 approx., and the area of each corner triangle is 1/2x^2 (1/2 base times height), which is 50/2=25. The length of the side of the enclosing square is 2*7.071+10=24.142.
If the radial sides of the octagon triangles are 10, the side of the octagon is 7.6537 (20sin(22.5 degrees)) and the side of the triangle is about 5.412 (sqrt(7.6537^2/2)). The side of the square is the side of the octagon plus 2*the side of the corner triangle, 7.6537+2*5.412=18.478.
The question doesn't specify what the measurement 10 is and doesn't specify what property of the triangle x is.