Definition of tangent of a given angle α :
tan( α ) = measure of the opposite side / measure of the adjacent side.
For the given case,
angle α = 35 °
measure of opposite side = x cm
measure of adjacent side = 12 cm
From definition of tangent,
tan(35 ° ) = x/12
Isolate and solve for x:
x=12 tan(35) = 12 * 0.7002 = 8.4 cm