Reassuring, Heartening
い adjective
Things that make your heart strong are reassuring and heartening to hear. You can practically feel your heart becoming stronger in your chest.
こころづよい
The readings for this word are both kun'yomi readings. You learned them when you learned the words 心 and 強い, so you should be able to read this on your own. Just note that つよい gets rendaku'd to づよい!
これは心強いデータですね！
This is heartening data!
プロのプレイヤーがパートナーになってくれるなんて、心強いです。
It's reassuring to have a professional player as my partner.
先生の助言は、私たち学生にとって心強いサポートとなりました。
The teacher's advice became a reassuring support for our students.