What is the equation of the line, if the slope is -1/2 and (-5,5) is on the line

Use the point-slope form of the  equation:-

y - y1 = m(x - x1)    where m=slope and (x1,y1) is a point on the line

Here we have

y - 5 = -1/2(x - (-5))
y - 5 = -1/2(x + 5)
y = -1/2x - 5/2 + 5
y = -1/2x + 5/2

this can also be written as

2y = -x + 5

or in standard form it is  x + 2y = 5