Part A: Since 85% of the marigold plants remain after each year, the equation for the marigold plants would be P=150(0.85)^x, with x being the amount of months. Since 8 sunflowers are removed from the initial 125 each month, then that equation would be P=125-8x, with x being the amount of months.
Part B: Just sub 3 in for the x in both equations, which will give you about 92 marigold plants left and 101 sunflower plants.
Part C: Enter both equations in to a scientific calculator and graph them. Then hit 2nd, trace, and 5 to find their intersection. The x-value of that intersection will be the number of years it takes for the amount of plants to be the same.