Im trying to figure out how to find the length of an equilateral triangle

All 3 sides of an equilateral triangle are the same.

Set up an equation with two sides equaling each other and solve for n:

2n +15/4 = 7n

Subtract 2n from both sides:

15/4 = 5n

Divide both sides by 5:

n = (15/4) / 5

n = 3/4

Now you have a value for n, replace n and solve for the length:

7n = 7(3/4) = 21/4 = 5 1/4

Each side is 5 1/4

e) none of the above because the measure of angle cba is 101°