From the left right-angled triangle you have this Pythagorean equation
x^2 + h^2 = 6^2. (1)
From the right right-angled triangle you have this Pythagorean equation
(6-x)^2 + h^2 = 4^2. (2)
Subtract eq(2) from eq(1). You will get
x^2 - (6-x)^2 = 36 - 16,
x^2 - 36 + 12x - x^2 = 20,
12x = 20 + 36 = 56.
=======> x = = .
The triangle is isosceles, with two sides of length 6. Then using the side with length 4 as the base, the height of the triangle will be the length of the other leg of a right triangle with one leg 2 (half of the 4) and hypotenuse 6. So the height is
Then the area of the triangle is one-half base times height:
Then, using your diagram with 6 as the base, the area of the triangle is one-half base times the altitude you show (call it h):
And then the unknown x we are looking for is the other leg of a right triangle with one leg h and hypotenuse 6: