hope you know that an equilateral triangle has sides length equal ,so in this case we need writing
4n=2n+10=7n-15
2n=10=7n-15
2n=10
n=5
10=7n-15
35=7n
n=35/7
n=5
so for n=5 the triangle will be equaliteral with sides length equal 20 units
hope helped