Engineering Mathematics, Linear Algebra 2015(1)

Consider the following 2x2 matrix A where two elements are unknown and are marked by a and b. The eigen values of this matrix are -1 and 7. What are the values of a and b?

$$ A = \begin{bmatrix} 1 & 4 \\ b & a \end{bmatrix} $$

1. a = 6, b = 4
2. a = 4, b = 6
3. a = 3, b = 5
4. a = 5, b = 3