Make y the subject,
y - 2x = -5
y = 2x - 5
Now that y is the subject, the coefficient of x is your slope. HOWEVER! We are finding the slope perpendicular to this. Use the rule:
m1 * m2 = -1
Where m1 is the slope and m2 represents the perpendicular slope. Sub in 2 (from previous step)
2 * m2 = -1
m2 = -1/2
Therefore, your slope perpendicular to y - 2x = -5 is -1/2!
Hope this helped! Ask me if there's any part of the working you don't understand :)