I assume that a represents the salary in thousands of dollars. So if we plug in 140, this represents $140,000 per annum. And P= 11% approx. So we know that the smallest annual income is less than $140,000.
If P=6%, then 0.0014a²-0.1529a+5.1=6, 0.0014a²-0.1529a-0.9=0. Using the quadratic formula we get:
a=(0.1529+√(0.02338+0.00504))/0.0028=114.81 approx.
So the minimum income is $114,800 approx.