Question

Sum of two rational number is -8 one number is 3by4 find other

Answer 1

let the other number be x
x+3/4=-8
x=-8-3/4
x=-27/4

Answer 2

Let the other number be x

so by the condition given in your question we get equation

x+3/4=-8

x=-8-3/4

x=-24-3/4

x=-27/4