Beide Brüche halt auf einen Hauptnenner bringen
a/c - (a + a·b)/(a + c)
= a·(a + c)/(c·(a + c)) - c·(a + a·b)/(c·(a + c))
= (a·(a + c) - c·(a + a·b)) / (c·(a + c))
= (a^2 + a·c - a·c - a·b·c) / (c·(a + c))
= (a^2 - a·b·c) / (c·(a + c))
= a·(a - b·c) / (c·(a + c))