To solve this with t-chart, simply pick some x-values and plug them into the original formula to calculate the corresponding y-values (see example below). Once you have that, then just look for the two zeros and average them. In this case, 3 and 6...the average of which is 4.5...so you know the x-value of the vertex is at 4.5. Then plug that into the original equation to get -6.75. Thus, the vertex is (4.5 , -6.75)