Explanation:
when x increases by 1. y decreases by 7.
e.g. from (-4, 7) to (-3, 0).
the slope is therefore -7/1 = -7.
the slope is always the factor of x in the equation.
y = -7x + b
b is the y-intercept (the y value when x = 0).
let's use e.g. (-3, 0) to get b :
0 = -7×-3 + b = 21 + b
b = - 21
so, the full equation is
y = -7x - 21