Answer:
110 + 5x (I'm not sure if you need the dollar signs...if you do, one goes in front of the 110 and one goes in front of the 5)
Explanation:
$20 + $90 = $110
Equation:
110 + 5x
110 represents the base cost of the turtle and the terrarium.
5 represents the cost of the turtle food
x represents the number of weeks that she is purchasing the food.