0.5 = 0.4y - 0.25x
Since we're solving for (y), we have to get that variable alone
0.5 = 0.4y - 0.25x
+ 0.25x + 0.25x
So add the 0.25x on both sides
0.25x + 0.5 = 0.4y
----------------- -------
0.4 0.4
Then divide everything by 0.4
0.625x + 1.25 = y
That's your answer (this is slope-intercept form, y = mx + b)