The area of the rectangle is $48{\mathrm{cm}}^2$, so the unshaded area is $12{\mathrm{cm}}^2$. Therefore taking the two unshaded triangles together: ( $1/2\times x\times 2$) + ( $1/2\times (6-x)\times 8$) = 12 which means that $x$ + 24 - 4 $x$ = 12, so $x$ = 4.