صنع مارك لوحة سوداء بت.setWidth Three أقدام. إنها أطول بمرتين من عرضها. ما مساحة اللوحة السوداء؟ 

Note: There seems to be a misunderstanding in your question. The sentence "It is 2 times as long as it is wide" cannot be directly translated to Arabic but rather it should be understood. A closer translation in meaning is " هي أطول بمرتين من عرضها". But, if you want a literal translation it's "هي ضعف longitudinallywat sheheen". But it doesn't make much sense in Arabic. I provided you with the more meaningful translation. 

The corrected sentence to be translated will be: "Mark made a chalkboard that is 3 feet wide. It is 2 times wide as it is long. What is the area of the chalkboard?" 

But, then the problem is illogical since the second sentence would suggest that the chalkboard is shorter in length than its width. Usually, the width is less than or equal to the length.

So, here is a more logical variant: 

"Mark made a chalkboard that is 3 feet wide. It is 2 times as long as it is wide. What is the area of the chalkboard?" 

And the proper translation in Arabic is:

صنع مارك لوحة سوداء بعرض 3 أقدام. إنها أطول بمرتين من عرضها. ما مساحة اللوحة السوداء؟ 

If you still want the literal translation, then it would read: 

صنع مارك لوحة سوداء بعرض ثلاثة أقدام. إنها طول ضعف عرضها. ما مساحة اللوحة السوداء؟ 

My recommendation is to use the first translation I gave you, as it is clear and makes sense in Arabic.
طول السبورة هو 2 × العرض البالغ 3 أقدام = 6 أقدام.
يمكننا حساب المساحة عن طريق ضرب العرض و الطول, لذا 3 أقدام × 6 أقدام = 18 قدمมربع.
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