Question

Whats the solution of y=2x-31

Answer 1

When there are two variables ('x' and 'y'), then you need two equations
in order to find a solution.

If you only have one equation with 2 variables, then there are an infinite
number of solutions. 

Here are a few solutions to this particular equation:

x =    0,   y =  -31
x =    1,   y =  -29
x =    2,   y = -27
x =    3,   y = -25
x =  15,   y =   -1
x =  20,   y =    9
x =  30,   y =   29
x =  40,   y =   49
x =  50,   y =   69
x = 100,  y = 169

Now that you have these 10 solutions,
there are only an infinite number of others.