Question

Y is the square of a certain whole number added to three times a number makes 90 find the number

Answer 1

?? can't quite make out what you're saying
y^2 + 3y = 90
?? if so, it won't be an integer ...

unless "a certain whole number" is not "a number"

Answer 2

Maybe your question is:

y is twice the square of a certain whole number added to 3 times the number gives 90. Find the number

If that is the question then:

2 y² + 3 y = 90

2 y² + 3 y - 90 = 0

The solution are:

y = - 7.5 and y = 6

- 7.5 is not whole number.

So y = 6