The fixed cost is f so c, cost=f+aq where we have to find the constants f and a.
But we have some figures: when q=10, c=45 and when q=20, c=80. We can write:
45=f+10a and 80=f+20a. Subtract the two equations: 35=10a so a=35/10=3.5.
So we now have a and that means we can find f: 45=f+35 making f=10.
Also, 80=f+70 and, again, f=10 so the equation is c=10+3.5q.
When q=35 c=10+3.5*35=10+122.5=132.50 Tsh.