This is because mathematics uses some theories that helps to change words to equations. for example, a tree of height 50m makes a shadow on the ground. an observer standing at angle of 30 degrees, calculate the distance of the man to the tree. we use the the formula, tan (30)=opp/adj
If slope of a line is m then slope of the perpendicular line is -1/m . so slope of perpendicular line is -3. then use this y-y0=m(x-x0) y+5=-3(x-2) y+5=-3x+6 y=-3x+1