Answer
$x=-1+\sqrt{11}$
Work Step by Step
The area of a trapezoid is the product of half the sum of the bases and its height.
Hence here: $0.5 (3x-1+x+9)x=20\\x(4x+8)=40\\x(x+2)=10\\x^2+2x-10=0\\(x+1)^2-1^2-10=0\\(x+1)^2=11\\x+1=\pm\sqrt{11}\\x=-1\pm\sqrt{11}$
But we know that $x$ is non-negative, thus $x=-1+\sqrt{11}$
