a) 910 : x=26
x= 910 : 26
x=35
b) 206-(3x+42)=13.5
206-(3x+42)=65
3x+42=206-65
3x+42=141
3.x=141-42
3.x=99
x=99:3
x=33
c) $3^{7}$ . $3^{9}$:$3^{14}$+$x^{2}$ =25
$3^{16}$ :$3^{14}$+$x^{2}$ =25
$3^{2}$+$x^{2}$ =25
$3^{2}$+$x^{2}$ =5²
$x^{2}$ =5²-$3^{2}$
$x^{2}$ =2²
⇒x=2