5x² + 2x - 3 < 0
5x² + 2x < 3
x² + 2/5 x < 3/5
Complete the square on the left side:
x² + 2/5 x + 1/25 - 1/25 < 3/5
(x + 1/5)² - 1/25 < 3/5
(x + 1/5)² < 16/25
Take the square root of both sides, keeping in mind that √(x²) = |x| for all x.
√((x + 1/5)²) < √(16/25)
|x + 1/5| < 4/5
This means
-4/5 < x + 1/5 < 4/5
-1 < x < 3/5
