The given values of abscissa and ordinate in the given are that which when substituted to the values of x and y in the equation will make the equation true. This means that the left and right-hand side of the equation will be equal. The established equations should have a basis of,
x + y = c
The equations are,
x + y = 6
Substituting x = 7 and y = -1,
7 + (-1) = 6 ; 6 = 6
The other two equations will be,
2x + y = 13 and,
x + 2y = 5
It is to be noted that countless of equations can be made with the given solution so long as the equation is valid.