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'Exactly Three-quarters' printed from https://nrich.maths.org/
The area of the rectangle is $48cm^2$, so the unshaded area is
$12cm^2$. Therefore taking the two unshaded triangles together:
($1/2\times x\times2$) + ($1/2\times(6-x)\times8$) $= 12$ which
means that $x + 24 - 4x = 12$, so $x = 4$.