Lucia tried to solve the system below. Lucia’s Work. What error did Lucia make? When Lucia solved for the value of x, she subtracted 2y from both sides. When Lucia solved for the value of y, she treated –2y as 2y. When Lucia solved for the value of x, she added 18 to both sides. When Lucia solved for the value of x, she treated –2(–9) as 18 instead of –18.