a)4x+5=x+13
4x - x = 13 - 5 ; 3x = 8 ; x = 8 / 3
b)11x+17=2x+21
11x - 2x = 21 - 17 ; 9x = 4 ; x = 4 /
c)3x-1/ 2=3/ 7-4x
21x - 7 - 12x² + 4x = 6
-12x² + 25x - 13 = 0
12x² - 25x + 13 = 0 Δ = 625 - 12·4·13 = 625 - 624 = 1
x = ( 25 - 1) /24 = 1 si x = ( 25 + 1 ) /24 =26 /24 = 13 /12