Since 125 represents a maximum heart rate (which has been generally defined as M), you must plug 125 into the equation to get 125 = 176-0.8A. You can solve it from here by subtracting 176 from both sides to get -51 = -0.8A, then dividing both sides by -0.8 to get A = 63.75. You can then round (it says approximately) to A = 64 as your final answer (Option 2).
