A PMF Proportional to the Value

A bakery stall records how many pastries each customer buys. The number bought, $X$, is a discrete random variable taking values only in $\{1, 2, 3, 4\}$.

The stall's model says the probability of each value is proportional to the value itself:

$P(X = x) = c\,x \qquad \text{for } x = 1, 2, 3, 4$

where $c$ is a constant, the same for all four values, chosen so that this is a valid PMF.

What is $P(X \ge 3)$?

Give your answer as a decimal probability, rounded to 2 decimal places.