We can model the cost and number of tickets by a linear equation of the form

Where c is the cost, t is the number of tickets.
m is the slope of the equation and b is the y-intercept.
First, let us find the slope which is given by

You can take any two pairs of values from the table.

The slope is 9.75 and the equation becomes

Now we need to find the y-intercept (b)
Choose any one pair of values from the table and substitute them into the above equation and solve for b.
Let's choose (12, 117)

The y-intercept is 0 so the equation is

Now to find the cost of 7 tickets, simply substitute t = 7 into the above equation

Therefore, the cost of 7 tickets is $68.25