Question

If you combine 340.0 mL of water at 25.00 °C and 140.0 mL of water at 95.00 °C, what is the final temperature of the mixture? Use 1.00 g/mL as the density of water

Answer 1

Use Q = M x C x delta T
Quantity of Heat = Mass x Specific Heat x Change in Temperature

Since the initial temperature is already given, you subtract the final temp from the initial temp to see the changes of temp that occurs (that's why it's known as delta T)
so,
M x C x (Tfinal - Tinitial1) = M x C x (Tfinal-Tinitial2)
specific heat of water is 4.184, therefore..
(340.0 g)*(4.184)*(x-25.00)=(140.0 g)*(4.184)*(95.00-x)
solve for x,
x is the final temperature that you're looking for.