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