A)
Because it uses the same line as RT, then you can get LN's slope by getting RT's slope.
Equation: (y₂ - y₁)/(x₂ - x₁)
Our case: (16-10)/(12-4) = 6/8 = 3/4
B)
Point-slope formula: y - y₁ = m(x - x₁)
We already have m, it's the slope. And we can choose whichever point from our graph, I'm gonna choose (4, 10)
Our case: y - 10 = 3/4(x - 4)
y - 10 = (3/4)x - 4(3/4)
y - 10 = (3/4)x - 3
y = (3/4)x + 7