Part A. Define the variables and the equation.
Let y be the money Ling wants to save and x be the weeks, then we can write

Part B. Solve the equation.
Since Ling wants $87, then y=87. By substituting this value into the last equation, we get

then, by moving 23 to the left hand side, we have

or equivalently,

Finally, by moving 4 to the right hand side, we obtain

Then, How long will it take her to get enough money to buy the tickets? 16 weeks