a) 2 - (x - 7,25) : 8(2/3) = 0
<=> 2 - x + 7,25 : 26/3 = 0
<=> 2 - x + 87/104 = 0
<=> x = 295/104

b) (1 + 2 + 3 + ... + 19) : x = 10
<=> [(1 + 19) + (2 + 18) + ... + (9 + 11) + 10] : x = 10
<=> (20 . 9 + 10) : x = 10
<=> 190 : x = 10
<=> x = 19