Concept: The question is based on the application of Triangle area
Solution:
Consider the triangle with the side of 12 units as base & height 3 units,
=>Area = ½ *12*3 unit square (1)
Consider the triangle with the side of 7 units as base & height as x units,
=>Area = ½* 7*x unit square (2)
From 1 & 2
½* 12*3 = ½* 7 * x (As we are calculating the area of the same triangle)
=>x = 36/7
option(c)