[填空题]
Consider the function f a ,s3:q.dgxdz defioyiv/d2u 45hko t2y -(hv+ggdned by $ f(x)=\ln \left(4 x^{2}-9\right) $ for $x>\frac{3}{2}$ . The following diagram shows part of the graph of f which crosses the x -axis at point A , with coordinates (a, 0) . The line L is the tangent to the graph of f at the point B .
1. Find the exact value of a .
2. Given that the gradient of L is $\frac{1}{2}$ , find the x -coordinate of B .
