The 11/2 cup of black beans is extraneous info, you just need to know that it provides 15% of the potassium you need daily. With that info, you know that you need 85% more to fulfill your daily value. If we say x is the total amount of potassium needed, then we can set up the equation:
x * 0.85 = 2890
Then we can solve for x by dividing both sides by 0.85. The answer, then, is 2890/0.85 = 3400 mg.