Find the point p on the line y=5x that is closest to the point (78 0)

We know that
the point p on the line y=5x that is closest to the point (78 0) will be on the line perpendicular to the line y=5x and go through the point (78 0)

step 1
find the slope m perpendicular to the line y=5x
m1=5
if two lines are perpendicular
m1*m2=-1
m2=-1/5

step 2
with m=-1/5 and the point (78,0) find the equation of a line
y-y1=m*(x-x1)-----> y-0=(-1/5)*(x-78)----> y=-0.20x+15.6

step 3
resolve the equation systems
y=5x
y=-0.20x+15.6

using a graph tool
see the attached figure

the solution is the point
x=3
y=15

the answer is
the point (3,15)

C. expression because any expression or equation may contain more than one variable