2.5x + 8.5y=90 what is the number for x

It will have infinitely many solutions depending upon value of y.

For eg:

Put y=10,

2.5x+8.5*10=90

2.5x=5

x=2

For y=0

x=90/2.5=36

and so on

You need a system of linear equations to get a single solution for x