The equation of a straight line is
y = mx + c
4x + 20y = -180
make 20y the subject of the formula
20y = -180 - 4x
20y = -4x - 180
divide all through by 20
20y/20 = -4x/20 - 180/20
y = -1/5x - 9
The answer is y = -1/5x - 9 where your slope is -1/5 and intercept is -9