Example 1 - no calculation required
The y-intercept can be read from the graph. Since the parabola opens downwards a is negative. Match this to answer choices. Only one answer choice has the matching y-intercept and negative a.
Example 2 - mental math
Step 1: -b/2a can be calculated using mental math. Eliminate answer choices C and D that do not match the value of x.
Step 2: When x = -1 is plugged in the equation, 12x = -12. This is greater than 6+4. Hence, y must be negative. This eliminates answer choice B.
Example 3 Question 1 - mental math
Step 1: -b/2a can be calculated using mental math.
Step 2: Since the value of x is small, this can be done using mental math.
Example 3 Question 2 - may be mental math
-16t can be factored out simplifying the equation to t - 2 = 0 Hence, t = 2.