The figure is a parallelogram so the facing sides have equal size which means :
DE = GF & DG = EF
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DE = 3x - 1 , GF = 2x + 11
DE = GF ==》 3x - 1 = 2x + 11
subtract sides minus 2x
==》- 2x + 3x - 1 = - 2x + 2x + 11
collect like terms
==》 x - 1 = 11
add sides 1
==》 x - 1 + 1 = 11 + 1
==》 x = 12
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As we found : DG = EF
DG = 2x - 3 So EF = 2x - 3
To find the size of EF, we need to put the value of x (which we found above) instead x.
EF = 2 × ( 12 ) - 3 ==》 EF = 24 - 3
==》 EF = 21
And we're done...