Find the positive value of $a$ that satisfies $\int _ { 0 } ^ { a } \left( 3 x ^ { 2 } - 4 \right) d x = 0$. [3 points]\\ (1) 2\\ (2) $\frac { 9 } { 4 }$\\ (3) $\frac { 5 } { 2 }$\\ (4) $\frac { 11 } { 4 }$\\ (5) 3