Can a linear equation have only one variable

In the polynomial below, what number should replace the question mark to produce a difference of squares? x2 + ? x - 36

The zoo collects $9.60 for every 24 tickets sold. how much will be collected for 400 tickets?

Which equation is true? i. 56 ÷ 4·7 = 82 ÷ (11 + 5) ii. (24 ÷ 8 + 2)3 = (42 + 9)2 neither i nor ii ii only i and ii i only