slope intercept form of a straight line equation is y = m * x + b.
m is the slope.
b is the y-intercept.
the slope is the same as he constant rate.
let y = the number of feet of grain left in the silo.
let x = the number of seconds.
you get y = -3.5 * x + b.
when x = s seconds, the equation becomes y = -3.5 * s + b.
b is the y-intercept which is the value of y when x = 0.
when x = 0, no seconds have elapsed and the silo is therefore at whatever height it was before you started to empty it.
that would be the value of b.
if we assume that the height of the grain in the silo was 150 feet, then the formula would become: y = -3.5 * x + 150
after s seconds, the formula would become y = -3.5 * s + 150.
if s was equal to 20, then the formula would become y = -3.5 * 20 + 150 which would then become y = -70 + 150 which would then become y = 80.
after 20 seconds, there would be 80 feet of grain left in the silo.