Question

write and equation of a line perpendicular to the line y=1/2x+10 and passes through (0,3)

Answer 1

if it is perpendicular you know that the gradient of the perpendicular line is the negative reciprocal of the line you are already given
for example if you had an equation
y=2x+3
a line perpendicular to that would have a gradient of
-1/2

Answer 2

figure out the gradient of the perpendicular line, then use the equation for the equation of a line, and sub in your values given ( the x and y coordinates)