Answer:
d = 9
Step-by-step explanation:
Use the slope formula to calculate m and equate to m = 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 )
3(d - 4) = 15 ( divide both sides by 3 )
d - 4 = 5 ( add 4 to both sides )
d = 9
