As the first line passes through the origin, that is point (0,0) and the through the point (4,-5), you can find the slope of this line with the formula

So, you have

Now, you can use the point-slope formulas to find the equation of the line in its slope-intercept form

Therefore, the equation of the first line and the answer of numeral b) is

On the other hand, the slopes of perpendicular lines satisfy the following equation

So, to find the equation of the second line that is perpendicular to the first, you can find its slope and then use the point-slope formula

Now, using the point-slope formula with the point (4,-5)

Therefore, the equation of the second line and the answer of numeral a) is

Graphically,