Bonjour !
a) 7x-6 = 4-3x
<=> 7x+3x = 4+6
<=> 10x = 10
<=> x = 10/10 = 1
b) 12-a = 18-3a
<=> 3a - a = 18-12
<=> 2a = 6
<=> a = 6/2 = 3
Voilà.