to find the height of a tree, a student 53 inches in height measures the length of the tree's shadow and the length of his own shadow. The student casts a shadow 63 inches in length and the tree casts a shadow 67 inches in length

It's a proportion...53/63=x/67

in other words...
st.height/st.shadow=tr.height/tr.shadow
Solve by cross multiplying...63x=53*67
so x=56.37inches..

Good Luck!