Solve for x
x=-1
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20=x+3\times 7
Multiply 5 and 4 to get 20.
20=x+21
Multiply 3 and 7 to get 21.
x+21=20
Swap sides so that all variable terms are on the left hand side.
x=20-21
Subtract 21 from both sides.
x=-1
Subtract 21 from 20 to get -1.
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