Answer:
x = 4 rounded to the nearest whole number (actually 4.0801)
Explanation:
The triangle formed by the sides of length 0.9x, 2.8x and 12 form a right triangle with the side of length 12 as they hypotenuse
By the Pythagorean theorem
c² = a² + b² where c = hypotenuse, a and b being the other two sides
Plugging in values we get
12² = (0.9x)² + (2.8x)²
144 = 0.81x² + 7.84x² = 8.65x²
Switching sides:
8.65x²= 144
x² = 144/8.65 = 16.6473
x = √16.6473= 4.0801
Rounded to the nearest integer, x = 4