First, find out the value of x to confirm if they are really true.
We know that all the interior angles of a triangle equal to 180 degrees.
4x+5x-8+3x+20=180
12x=180-12
12x=168
x=168/12
x=14
Put in these values of to find if AB=AC.
(4×14)(3×14+20)=(4×14)(5×14-8)
Solve for LHS first,
(4×14)(3×14+20) = 56×62
=3472
Solve for RHS next,
(4×14)(5×14-8) = 56×62
=3472
Since LHS=RHS, AB is also equal to AC.
Hope I helped :)