Simplify the expression $$\frac{4z}{z-1} + \frac{z}{z+3}$$ a...

Question Analysis
This problem assesses the ability to add rational expressions and simplify the result.

Key Concept Explanation
To add rational expressions, find a common denominator and combine the numerators. The common denominator for and is .

Step-by-step Solution
1. Find a common denominator:
2. Rewrite each fraction with the common denominator: