Differential equations help , rate of change

A mathematician is selling goods at a car boot sale. She believes that the rate at

which she makes sales depends on the length of time since the start of the sale,

t hours, and the total value of sales she has made up to that time, £ x.

She uses the model

$\displaystyle \frac{dx}{dt} = \frac{k(5-t)}{x} $

where k is a constant.

Given that after two hours she has made sales of £96 in total,

(a) solve the differential equation and show that she made £72 in the first hour of

the sale.

The mathematician believes that is it not worth staying at the sale once she is making sales at a rate of less than £10 per hour.

(b) Verify that at 3 hours and 5 minutes after the start of the sale, she should have

already left.

$\displaystyle \int x dx = \int k(5-t) $

$\displaystyle \frac{1}{2}x^{2} = 5kt - \frac{1}{2}t^{2} + c $

t= 0, x =0 , c = 0

t = 2 , x = 96 k = 576

I really suck on part'b', how would I go about it?

thank you