The formula of a slope:
[tex]m=\dfrac{y_2-y_1}{x_2-x_1}[/tex]
We have the points (-10, 2) and (-8, k) and the slope m = 3. Substitute:
[tex]\dfrac{k-2}{-8-(-10)}=3\\\\\dfrac{k-2}{2}=3\qquad\text{multiply both sides by 2}\\\\k-2=6\qquad\text{add 2 to both sides}\\\\\boxed{k=8}[/tex]
