Each adult ticket is 6$, so if you sell [tex] a [/tex] adult tickets you gain [tex] 6a [/tex] dollars.
Similarly, each student ticket is 2$, so if you sell [tex] s [/tex] adult tickets you gain [tex] 2s [/tex] dollars.
So, the total is
[tex] T = 6a+2s [/tex]
Since, in your case, a=90 and s=50, the expression becomes
[tex] T = 6\times 90+2\times50 = 540 + 100 = 640[/tex]