If x equals 5 and y equals 8 then what is the answer to this equation? 5x-4y= 1x + 3y

Note  that 

5x - 4y =

5(5)  - 4(8)   =  

25 - 32  =

-7

But

1x + 3y  =

1(5) + 3(8) =

5 + 24  =

29

So........this "equation" is not true for these values of x and y

You cannot calculate directly,

5x - 4y = 1x + 3y

5x - 1x = 3y + 4y

4x = 7y ...............(1)

suppose if we substitute the value of x as 5 in equation (1), we get,

20 = 7y

y = 20/7

OR we can substitute, y as 8 in equation (1), we get,

4x = 56

x = 56/4 = 14