Cumulative distribution function - checking whether my answer is correct

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A random variable X has a cumulative distribution function

$$F(x) = {Ax+B}$$
if

0 < x < 3 (x is greater than or equal to xero)

0 if x < 0 (x is smaller or equal to zero)

1 if x > 3 (x is greater than or equal to 3)

The question is find the constants A and B and find the pdf(x)

What I have done: evaluated Ax + B at x=0 which gives just a B

evaluated Ax + B at x=3 which gives 3A + B

Taking these two together, equated B to zero and 3A + B to 1 which gives A = 1/3

Is this the correct way?