7 and 41/65 units
Working
The area of a triangle is found by using the formula;
A=1/2bh where A is area, b is base and h the height
substituting for given values;
12 2/5=1/2*3 1/4*h
Rewriting the values as improper fractions;
62/5=1/2*13/4*h
62/5=13/8h
h=62/5 *8/13
h=496/65
h= 7 41/65 units