Trouble finding dx/dt while dx/dt = x(t)

[StudyOriented]

Hello,

First of all, sorry if my English is bad, that because i am not from an English-speaking country.
But, i think you could understand what i say, because mathematics is universal.

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Well, my brother give me an example of his calculus exam test. He's already in his first year the university, while i am still High School.

He said that it will use the natural logarithm ( ln )

dx/dt= k x
dx/x = k dt
integral both side get;
ln x = ∫ k dt
= k (t) + C

x = (e^(kt)) (e^C) and e^C is constant

and that applied to the question below.

There is a tank full of water 120 Litres.

Saltwater from a pipe goes to the tank with the flowing 4 Litres per second and the composition of the saltwater is 1 gram per Litre.

The tank always stir the water inside it automatically.

And the stirred water inside it then flowed out from another pipe flowing 6 Litres of stirred water out per second.

The question is, how many grams of salt left at the time of "t"?

As far as i calculate the formula for the salt is

dx/dt = 4 - (6x/120-2t)

the trouble is there is still an x

but how to apply the formula that stated before? ( x = (e^(kt))(e^C) )

and how to find the pure formula of x , x(t)=?