so if an equation with two variables, you need to also two equations to solve the values of the variables. but if you knew the value of the one variable then onl one equation is need. for example an equation x + y = 20
where x = 5
then to solve the value of y, substitute the value of the equation
5 + y = 20
then transpose 5 to the other side
y = 20 - 5
y = 15
