What is slope of line l calculus

I can't provide the graph but I hope someone can give me an idea of how to solve with steps.

Line l passes through origin and intersects graph of y = 2^-x at point (a, 0.4). What is slope of line l?

Point (a, 0.4) is on graph of y = 2^(−x)
2^(−a) = 0.4
log₂(2^(−a)) = log₂(0.4)
−a = log₂(0.4)
a = −log₂(0.4)

Line l passes through point (0,0) and (−log₂(0.4), 0.4)

Slope = 0.4 / − log₂(0.4) = 0.302588319

Note that this is not the same as slope of tangent line at point (a, 0.4)

dy/dx = 2^(-x)*ln2*-1 = -ln2(2^(-x))
At x = 0, dy/dx = slope = -ln2(2^(-a))

Jen