Question 841871
You will need the densities of sulfuric acid solutions.  Check this:
http://www.chembuddy.com/?left=CASC&right=density_tables


60%_________1.5031 g/cc
90%_________1.8197
Unless you can find another data source (like maybe a handbook), you might try estimating the 75% concentration density, {{{(1.615+1.7323)/2=1.6736}}} g/cc, only an estimate or prediction.


You did not say if your concentrations are w/w or w/v.  You probably expect to use percent w/v.  But the data in the reference source is percents w/w.


Let x = volume, ml, or the 60%
Let y = volume, ml, of the 90%
And assuming volumes are simply additive:


Pure sulfuric acid:  {{{x*1.5031*(0.60)+y*1.8197*(0.90)}}} grams of sulfuric acid.
Volume of 75% solution:  {{{(1.6736)(x+y)}}} grams of solution.


Compose the percentage equation to be all as percent by weight.
{{{highlight((x*1.5031*(0.60)+y*1.8197*(0.90))/((1.6736)(x+y))=0.75)}}}


The volume sum equation is just {{{highlight(x+y=8000)}}}, using milliliters.


Those two red-highlited equations are the system to solve, for x and y.