Definition: The y-intercept is the point where the graph crosses the y-axis, which occurs when x = 0.
Step 1: Set x = 0 in the equation y = x^2 + 3x.
Step 2: Compute y: y = 0^2 + 3(0) = 0 + 0 = 0.
Conclusion: The y-intercept is y = 0, corresponding to the point (0, 0). Final answer: 0.