s= soft taco
d= double deckers
$11.25= 3s+3d
$10.00= 4s+2d
first eliminate 1 variable, lets say d ,to do this we must cancel them out, to do this multiply the top by -2 and the bottom by 3
-2 ($11.25= 3s+3d) -$22.50= -6s-6d
3 ($10.00= 4s+2d) which becomes... $30.00=12s+6d
then the -6d and the positive 6d cancel out and you are left with 1 variable
after this add the remaining parts to get...
$7.50=6s divide by 6, then put s back into one of your original equations and solve for d then you have found how much each item costs and are finished with the system of equations