i forgot how to do this, i need help please

Answer:
[tex]\frac{48}{13}\approx 3.7[/tex]
Step-by-step explanation:
Two triangles are similar by AAA ( Angles B and B' are right angles, the two angles in C are congruent because opposite by the vertex, and the third by difference). Corresponding sides are in proportion. In particular [tex]x:6=8:13 \leftrightarrow 13x=6\cdot8 \rightarrow x = \frac{48}{13} \approx 3.7[/tex]