Ok, so the population wanted is 500. We plug in and now we have:
500 = 3t + 10√t + 120, solve for t
I suggest bringing the 3t and then 120 to the left side with subtraction then squaring both sides as your first few steps to solving for t.
A large pond is stocked with fish. The fish population P is modeled by the formula P = 3t + 10√t + 120, where t is the number of days since the fish were first introduced into the pond. How many days will it take for the fish population to reach 500?