you plug in the y .8 to the 8 that is 6.4 them you subtract 40 by 6.4 that is 33.6 then you divide that by 4 is 8.4

Substitute in 0.8 for y. [latex]4x+8y=40 \\ \\ 4x + 8(0.8) = 40 \\ \\ 4x + 6.5 = 40 \\ \\ 4x = 40 - 6.4 \\ \\ 4x = 33.6 \\ \\ x = 8.4 \\ \\ [/latex] The final result is: x = 8.4.