If (5x-12)(x+4)=0 then x=?

You have to do the FOIL method

multiply the numbers that come first together
5x • x = 5x²
then multiply the outer numbers together
5x • 4 = 20x
then multiple the inner numbers together
-12 • x = -12x
then multiply the last numbers together
-12 • 4 = -48

then add the answers together
5x² + 20x – 12x – 48 = 0
then put together the x coefficients

(20x–12x)= 8x

so we have
5x² + 8x – 48=0
you can then use a calculator to solve
and the answer will be
X1= 2.4 X2=-4

That they are numbers and related to each other.