a) 2x - 138 = 23 . 32
2x - 138 = 8 . 9
2x - 138 = 72
2x = 72 + 138
2x = 210
x = 210 : 2
x = 105
b) 231 - ( x - 6 ) = 1339 : 13
231 - ( x - 6 ) = 103
x - 6 = 231 - 103
x - 6 = 128
x = 128 + 6
x = 134
c) ( x + 1 ) + ( x + 2 ) + .... + ( x + 100 ) = 5750
( x + x + ... + x ) + ( 1 + 2 + ... + 100 ) = 5750
=> 100x + 5050= 5750
100x = 5750 - 5050 = 700
=> x = 700 : 100 = 7