Serena wants to solve the following system of equations in the most efficient way.

2 x + 3 y = 18. x + 7 y = 31.

She plans to solve for x in the first equation as her first step since both 2 and 3 can be divided into 18. Why is Serina mistaken? Serina should have solved for x in the second equation because it has a coefficient of 1. Serina should have solved for y in the first equation because dividing by 3 instead of by 2 would give a smaller number in the solution. Serina should have solved for y in the second equation because it has the largest coefficient. Serina should have solved for y in the first equation because the division step will be easier since 18 is divisible by 3.