If there is an isosceles triangle and two equal.....?

Yes; the only angle we mind is the different one, so its HALF is one of the angles of the two opposite) rectangule triangles that forms the isosceles one. Then, using trigonometric functions, the sin of that angle multiplied by one of the sides of your triangle is precisely half or the unknown side. So, 8.4 sin (52/2) = 8.4 sin 26 = 3.68. The side you're looking for its lenght is double this figure, so 3.68*2 = 7.36