To make ∠ADC is a right angle, value of x will be 7.
From given figure, it is observed that Angle ADC is combination of two angles ADB and BDC.
∠ADC=∠ADB+∠BDC
Since, ∠ADC is right angle.
So, ∠ADC=90 degree
90=∠ADB+∠BDC
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ANSWER: 21 mm
EXPLANATION: valid