Why is the derivative of y = 2 (50-x) not right? The correct answer is - 2 and I get the answer by y = 2U, u = 50-x

Why is the derivative of y = 2 (50-x) not right? The correct answer is - 2 and I get the answer by y = 2U, u = 50-x

y=2(50-x)
y=2u ,u=50-x
y'=2u',u'=(50-x)'=-1
y'=2*(-1)=-2