Substitute y = a2 and get a quadratic expression.
a4 - 13a2 + 3b = y2 - 13y + 3b = (y + A)(y + B) = (a2 + A)(a2 + B)
AB = 3b
A + B = -13
However, without a value for b, there is not much more we can do. I am convinced that the 3b is a typo, and that the problem really has 36 instead of 3b, as suggested by Guiyou Q. Note how that gives a very straightforward factorization.