I was confused about should I make sales revenue as "x" or make wages expense as "x"

(b) The total manufacturing cost for two levels of activity is given below: Level 1 Level 2 Units produced 2,000 5,000 Total cost $15,000 $22,500 Required Assuming the variable cost per unit and the total fixed costs remain constant over the relevant range and time period, compute the total fixed cost.(a) The following information on sales revenue and wages expense was collected over a six- month period: Month Sales ($'000) Wages ($'000) January 240 141 February 150 97 March 170 107 April 180 111 May 200 124 June 210 128 Required Using the high-low method, determine the formula for calculating wages expense within the relevant range. A