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DontPanic
The missing number is 23 because,
there is 3 feet in 1 yard
so, 7x3=21
21+2=23 
so 7 yards and 2 feet = 23 feet
NeonX
The unknown number is 23 feet because in 1 yard, there are 3 feet. So, 3 multiplied by 7 yards is 21 feet. 21 feet and the 2 other feet combined is 23 feet in total.