Use midpoint theorem :- The line segment in a triangle joining the midpoint of two sides of the triangle is said to be parallel to its third side and is also 'half of the length of the third side'.
So,
2x = ½ (x + 9)
2x = ½(x) + ½(9)
2x - ½x = 9/2
3/2x = 9/2
x = 9/3
x = 3
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Hope it helps ⚜