To simplify calculations,

for (1), begin by factorising 35 to write it as a multiple of the single-digit terms, 5 and 7, may be simpler ways though.

For (2), 9 is a power of 3, 4 is a power of 2 and 6 is a multiple of 3 and 2.

For (3), 4 and 8 are powers of 2, 12 is a multiple of 2, but this is a quadratic in disguise.