Answer:
d = 9
Step-by-step explanation:
Calculate the slope m of line AB and equate the result to 3
m = [tex]\frac{y_{2}-y_{1} }{x_{2}-x_{1} }[/tex]
with (x₁, y₁ ) = A(4, 6) and (x₂, y₂ ) = B(d, 21)
m = [tex]\frac{21-6}{d-4}[/tex] = [tex]\frac{15}{d-4}[/tex] = 3 ( multiply both sides by d - 4 )
15 = 3(d - 4), that is
15 = 3d - 12 ( add 12 to both sides )
27 = 3d ( divide both sides by 3 )
9 = d
