To solve the equation 2/5x + 3/14 = 1/7 - 4/21, we need to find a common denominator for the fractions on both sides of the equation.

The least common denominator (LCD) for 5, 14, and 21 is 70.

Multiplying the equation by 70, we get:

70 * (2/5x + 3/14) = 70 * (1/7 - 4/21)

Simplifying both sides of the equation:

(70 * 2/5)x + (70 * 3/14) = (70 * 1/7) - (70 * 4/21)

28x + 15 = 10 - 20

Combining like terms:

28x + 15 = -10

Subtracting 15 from both sides:

28x = -25

Dividing both sides by 28:

x = -25/28

Therefore, the solution to the equation 2/5x + 3/14 = 1/7 - 4/21 is x = -25/28.