Translate the question into an equation. Let x equal the number, and then 2x = x – 4. Solving for x, we find that x = -4. This is (C).

Sonal needs s soil samples. If according to the question, he must have more than 6 samples, then s > 6. Also according to the question, he may have no more than 13 samples, so s ≤ 13. Combining these two expressions, we find that 6 < s ≤ 13. This is (C).

The graph of f(x) has a y-intercept at y = 3. Because of this, we know that when y = 3, x = 0. f(x) must then satisfy the condition that f(0) = 3. This is true only for (A) and (B). Alternatively, by recognizing that each equation is in the slope-intercept form: f(x) = y = mx + b, where b is the y-intercept, we can reach the same conclusion. Next, notice that the slope of the line is positive. That is, as the value of x increases, so too does y. Returning to the slope-intercept form, m gives the slope of the line. Only (D) has a positive coefficient (m). Choice (D), then, is the correct function.

If x + y = 0, then x = -y. Using this relationship and substituting into the expression x – y, we find that x – y = -y – y = -2y. This is (A).

This question requires factoring the expression 2x² – 6x – 8. Begin by factoring 2 from the expression: 2(x² – 3x – 4). This expression is further factorable, giving 2(x – 4)(x + 1), which is (A).