Question 442617
Since you mention Pythagoras, it's a right triangle.
2x is the hypotentuse, the longest side.
{{{(2x)^2 = 15^2 + (2x-3)^2}}}
{{{4x^2 = 225 + 4x^2 -12x + 9}}}
225 -12x + 9 = 0
12x = 234
x = 19.5
-----------
The sides are 39, 36 & 15