Weare dealing with similar triangles given by the image:
Because we have two triangles with the same three angles, including an angle opposite by the vertex, and the angle marked as equal. Then, the third angle must be equal as well.
So we can make the following proportion: 8 / b = 10 / 4
and solve for "b" by crossmultiplying and then isolating b:
32 = 10 b
b = 32/10
b = 3.2
We can find a also via proportions:
a/ 4 = 9 / 10
multiply by 4 both sides:
a = 36 / 10
a = 3.6