Alright, here is how you determine how much equity (the % of the time you will win) and your pot odds (the relationship of your opponents bet to what you stand to win if you call).

On the turn you have 15 outs if you believe your opponent has an eight here. Those are 9 diamonds and the 6 straight outs that are NOT diamonds. This is where people typically mis-count by counting the same card twice.

There are 46 cards and you have 15 outs, making you hit just about 1 in 3 times, or 1:2 in ratio form, 1/3rd in fraction form.

Basically for every dollar you call with you need to see $2 in return. Therefore your opponent's bet here is too big for you to call. You are calling $1.74 to win $2.92. You would need to win $3.48 to make this call break-even.