4x^2-25<0
add 25 to each side
4x^2-25+25<0+25
4x^2 <25
divide by 4 on each side
x^2 <25/4
take the square root of each side
sqrt(x^2) < sqrt(25/4)
sqrt(x^2) < +sqrt(25/4) and sqrt(x^2) >-sqrt(25/4) flip the inequality when taking the negative
x<5/2 and x>-5/2
-5/2 < x <5/2