First of all, expand the left hand side:
Sum like terms on the right hand side:
As you can see, the equation turned to be an identity: it doesn't matter which value for x you are choosing: you're evaluating the same expression in both sides, so they will always be the same.
In other words, you're asking: which value for x should I choose so that -18x-12 and -18x-12 evaluate to the same number? Well, obvisouly any value for x will do.
So, there are infinitely many solutions to this equation, namely ever real number is a solution of this equation.
If A(t) is the amount of salt in the tank at time t, then A(0) = 50 g and
A'(t) = (1 g/L)*(3 L/min) - (A(t)/120 g/L)*(3 L/min)
Multiply both sides by , so that the left side can be condensed as the derivative of a product:
Integrate both sides and solve for A(t).
Given that A(0) = 50 g, we have
so that the amount of salt in the tank at time t is