Explanation:
first of all, remember, the sum of all angles in a triangle is always 180°.
now, we see that both bottom angles are 45°.
that means
180 = 45 + 45 + top-angle
90° = top-angle
aha ! we are dealing with a right-angled triangle, that is also isoceles (both legs are equally long, because the angles with the baseline are equal).
via Pythagoras
c² = a² + b²
where "c" is the Hypotenuse (the side opposite of the 90° angle), "a" and "b" are the legs.
in our case both legs are 7×sqrt(2) units long.
so,
baseline² = (7×sqrt(2))² + (7×sqrt(2))² = 49×2 + 49×2 =
= 98 + 98 = 196
baseline = sqrt(196) = 14 units
x is now the height of this triangle, and because of the isoceles form, it splits the baseline exactly in half.
so, one side of the baseline from x is 14/2 = 7 units.
and now Pythagoras for that sub-triangle :
(7×sqrt(2))² = 7² + x²
98 = 49 + x²
49 = x²
x = sqrt(49) = 7 units
the area of the triangle is
baseline × height / 2
in our case
14 × 7 / 2 = 7×7 = 49 units²