Problem 165

### Step-by-Step Solution

#### Part (1): Find the values of $m$ and $n$

Given that $|2m+n|$ and $\sqrt{3n+12}$ are opposite in sign, we can deduce that their sum is zero because one is positive and the other is negative (or vice versa), but their magnitudes cancel each other out. Therefore, we have:

$$|2m+n| + \sqrt{3n+12} = 0$$

Since both $|2m+n|$ and $\sqrt{3n+12}$ are non-negative (absolute value and square root ensure non-negativity), the only way their sum can be zero is if both are individually zero. This gives us two equations:

1. $|2m+n| = 0$
2. $\sqrt{3n+12} = 0$

From equation (1), we get:

$$2m + n = 0$$

From equation (2), squaring both sides to eliminate the square root, we get:

$$3n + 12 = 0$$

Solving these equations, we find:

- From $3n + 12 = 0$, we get $n = -4$.
- Substituting $n = -4$ into $2m + n = 0$, we get $2m - 4 = 0$, which gives $m = 2$.

Therefore, the values of $m$ and $n$ are $m = 2$ and $n = -4$.

$$\boxed{m = 2, n = -4}$$

#### Part (2): Solve the equation in terms of $x$

Given $n = -4$ and $m = 2$, we substitute these values into the equation $mx^2 + 4nx - 2 = 0$:

$$2x^2 + 4(-4)x - 2 = 0$$

Simplifying, we get:

$$2x^2 - 16x - 2 = 0$$

Dividing the entire equation by 2 to simplify further:

$$x^2 - 8x - 1 = 0$$

To solve this quadratic equation, we complete the square:

$$x^2 - 8x + 16 = 17$$

This gives us:

$$(x - 4)^2 = 17$$

Taking the square root of both sides:

$$x - 4 = \pm \sqrt{17}$$

Solving for $x$, we get two solutions:

$$x = 4 \pm \sqrt{17}$$

Therefore, the solutions for $x$ are:

$$\boxed{x_1 = 4 + \sqrt{17}, x_2 = 4 - \sqrt{17}}$$