Bonjour Kitchie

y = -x/3 +1
y = 3/4 x + 3/4

équivaut successivement à:
y = -x/3 +1
-x/3 +1 = 3/4 x + 3/4

y = -x/3 +1
-x/3 - 3/4 x = 3/4 - 1

y = -x/3 +1
-13/12 x = - 1/4

y = -x/3 +1
x = 3/13

y = 12/13
x = 3/13

A toi de tout reprendre, bon courage ...