Since both triangles are right triangles, we can apply the Pythagorean theorem:
c^2 = a^2 + b^2
Where:
c = hypotenuse ( the longest side )
a & b = the other 2 legs of the triangle
for the left side triangle ( T1 )
9^2 = 6^2 + y^2
Let's label "y" the missing side on triangle 1 , which is a common side for both triangles.
81 = 36 + y^2
81 - 36 = y^2
45 = y^2
√45 = y
Now, with triangle 2:
x^2 = 8^2 + (√45)^2
Solve for x
x ^2 = 64 + 45
x^2 = 109
x = √109 = 10.44