Use substitution.

Use the distributive property, then combine like terms.

2i + 4j + t(−i −7j)
= 2i + 4j −ti −7tj
= 2i −ti + 4j −7tj

If (0, 10) is on the line, then 0i + 10j is possible.

(2 − t)i + (4 − 7t)j = 0i + 10j

2 − t = 0 4 − 7t = 10

Does t have the same value in both equations?
If it is on the line, then it should.