integration – Splitting a single integral

After performing a dot product inside the integral this result was obtained. What rule was used to split this integral into two integrals in the last step?

$$int_a^{b} textbf{A} cdot textbf{dl} = int_a^{b} (xy dx + (3x-y^2) dy) = int_a^{b} xy dx + int_a^{b} 3x-y^2 dy $$