Equilateral triangle = all sides are equal

2x-7=y+5 ---- Eqn 1
x+y-9=y+5 -----Eqn 2

From eqn 1:

2x-7=y+5
y=2x-7=5
y=2x-12

Subst into eqn 2:

x+(2x-12)-9=2x-12+5
x+2x-12-9=2x-12+5
3x-21=2x-7
3x-2x=-7+21
x=14

Subst into y=2x-12

y=2x-12
y=2(14)-12
y=28-12
y=16

2x-7=2(14)-7
=21 cm
y+5=16+5
=21cm
x+y-9=14+16-9
=21cm

Answer: Each sides of the triangle is equal to 21cm, since it is an equilateral triangle (all sides are equal).

See photo. Understand the properties of equilateral triangle first and use substitution method since there are 2 unknowns (x and y). Need to substitute one (in this case x) in order to find the other first (in this case y). Once you know one of it, you can find the other by substituting back into the equation. Then substitute both unknowns (x and y) in order to solve the whole equation and get to the answer. In this case, the answer is correct that all sides is the same length since it is an equilateral triangle where all sides are equal.

Hope this helps. :)